Oregon's CAT Short-Period Filing Rule: What Businesses Starting or Closing Mid-Year Need to Know

You shut down your Oregon business in June, or you launched it in September. Either way, your first or last tax year is only a few months long — and that short year comes with its own Corporate Activity Tax return, its own deadline, and, in most cases, a smaller version of the familiar $1 million threshold. Get any of those three wrong and you face penalties on a tax you might not even have owed for a full year.

In April 2026 the Oregon Department of Revenue rewrote the rule that governs these partial-year returns — OAR 150-317-1015, effective May 1, 2026 — and it changed more than the fine print. There is a new due date, a formal day-count formula for prorating all three CAT thresholds, a carve-out for brand-new and closing businesses, and an explicit estimated-payment schedule for short years. This guide walks through each piece and what to do about it.

How Oregon's Corporate Activity Tax Works in 60 Seconds​

If you are new to the CAT, here is the shape of the tax before we get to the short-year wrinkles:

• It is a gross-receipts tax, not an income tax. The CAT applies to your Oregon commercial activity — roughly, Oregon-sourced gross receipts — with no deduction for expenses except a specific subtraction described below. You can owe it in a year you lost money.
• It hits every entity type. Corporations, S corporations, partnerships, LLCs, and sole proprietorships are all in scope. There is no small-entity exemption; the only question is how much Oregon commercial activity you had.
• The math is $250 plus 0.57 percent of taxable commercial activity above $1 million. Only taxpayers with more than $1 million in taxable Oregon commercial activity owe anything.
• There are three separate thresholds. You must register within 30 days of reaching $750,000 in Oregon commercial activity for the year; you must file a return if you top $1 million in Oregon commercial activity; and you owe tax if your taxable commercial activity exceeds $1 million after exclusions and the subtraction.
• The subtraction is 35 percent of the greater of your cost inputs or your labor costs, apportioned to Oregon, with compensation to any single employee capped at $500,000 a year.
• The money funds schools. CAT revenue flows to the Fund for Student Success for K-12 and early education spending — about $3.1 billion expected for the 2025–2027 biennium.

Normal annual CAT returns are due on the 15th day of the fourth month after your tax year ends — April 15 for calendar-year filers — and taxpayers expecting $5,000 or more in liability must make quarterly estimated payments. Short years follow different rules, which is where the amended regulation comes in.

What Counts as a Short Tax Period​

Under the amended rule, a short tax period is simply a period of less than 12 months. Four situations create one:

1. You start business mid-year. Your short period runs from the day you began business through the day before your next taxable year starts.
2. You stop business mid-year. Your short period runs from the day after your prior taxable year closed through the day you dissolved or ceased business.
3. You change your accounting period. If the IRS approves a federal accounting-period change, Oregon follows it for CAT purposes, and the stub left behind is a short period.
4. Your unitary group gets a new designated entity with a different tax year. Unitary groups file CAT as a single taxpayer through one designated member; swapping in a designee on a different year forces a short-period return.

Notice what is not on the list: a slow year. Doing business all twelve months but earning little is a full tax year with a small number on it, not a short period. The short-period rules turn on time, not revenue.

The New Due Date: The 15th Day of the Fifth Month​

The headline change is the filing deadline. Short-period returns are now due on or before the 15th day of the fifth month following the end of the short tax period — one month later than the standard annual deadline.

A business that ceases operations on June 30, 2026, for example, counts forward five months — July, August, September, October, November — and files its short-period return by November 15, 2026. A business whose stub year ends September 30 files by February 15. If the deadline lands on a weekend or holiday, it rolls to the next business day, consistent with Oregon's general deadline rule.

There is one exception, and it matters in acquisitions: a taxpayer that joins a unitary group mid-year must file its short-period return by the earlier of (a) the due date its own twelve-month return would have had if it had never joined the group, or (b) the due date of the unitary group's twelve-month return. If either the buyer or the seller is on a fiscal year, compute both dates — the earlier one wins.

Oregon offers a seven-month filing extension for CAT returns, automatic if you hold a federal extension, but an extension to file is never an extension to pay. Interest starts accruing the day after the original due date whether or not you extended.

Prorating the Thresholds: The Day-Count Formula​

Here is the part that surprises owners the most. For most short-period filers, the $750,000 registration threshold, the $1 million tax-rate threshold, and the $1 million filing threshold all shrink in proportion to the length of the short year:

Prorated threshold equals the full threshold multiplied by the number of days in the short period divided by 365, with the percentage carried to four decimal places.

The rule's own example makes the mechanics concrete: a taxpayer filing for June 1 through March 31 — a 304-day period — computes 304 divided by 365, or 0.8329. Its registration threshold becomes $750,000 times 0.8329, or $624,675, and both $1 million thresholds become $832,900.

Work through what that means for a business with a mid-year accounting change and $900,000 in short-period Oregon receipts. For a full year, $900,000 would mean no filing obligation at all. In a 304-day short year, the filing threshold is $832,900 — so the business must file, and must measure its tax against that lower line. Partial years pull the CAT's tripwires closer.

The Big Exception: New and Closing Businesses Keep Full Thresholds​

Now the twist that saves most readers: newly formed taxpayers that begin doing business mid-period, and taxpayers that cease business mid-period, do not prorate on that first or final short-period return. They use the full $750,000 and $1 million thresholds even though they operated only part of the year.

This is deliberate. A startup that opens its doors in October should not face a $200,000-scale filing threshold just because it existed for ten weeks, and a business winding down in March should not trip registration on a fraction of the normal trigger. The proration rule is aimed at continuing businesses with stub years — accounting-period changes and designated-entity swaps — not at births and deaths.

The practical upshot: if your short year exists because you started or stopped, you almost certainly have less to worry about than the formula suggests. If it exists because you changed your tax year, run the day count carefully.

The Subtraction Gets Prorated Too — Partially​

The 35 percent subtraction survives into short years, but with short-year adjustments:

• The subtraction base is tied to the short period. Only cost inputs and labor costs associated with the commercial activity reported in the short period count. You cannot drag a full year of expenses into a stub return.
• The $500,000 per-employee labor cap is prorated by days. In the rule's 304-day example (0.8329), the most compensation you can count for any single employee is $500,000 times 0.8329, or $416,450, before taking 35 percent of total eligible labor costs.
• Cost inputs follow your federal method. You determine cost inputs the same way you compute cost of goods sold for federal taxable income — no special Oregon costing method to learn, but no improvising either.

The computation order stays familiar: take the greater of eligible short-period cost inputs or eligible short-period labor costs, multiply by 35 percent, and subtract the result (along with exclusions) from Oregon commercial activity before comparing against the $1 million tax-rate threshold. In the rule's example, $200,000 of cost inputs beat $175,000 of labor costs, producing a $70,000 subtraction.

Estimated Payments on a Short-Year Clock​

If you expect $5,000 or more in CAT liability for the short period, quarterly-style estimated payments apply — compressed to fit the stub. The amended rule lays out the schedule by length of the period:

Two traps hide in this table. First, the trigger is expected liability of $5,000 or more — you owe estimates based on your forecast, regardless of when during the period you cross $1 million in activity. Second, each installment after the first is due with the return, not counting extensions. Filing on extension does not buy your estimates more time.

Miss an installment and Oregon assesses a 5 percent quarterly underpayment penalty per missed payment, on top of a 5 percent failure-to-pay penalty on tax unpaid by the original due date and a 20 percent failure-to-file penalty if the return lands more than three months late. Fail to register on time and a separate $100-per-month penalty (capped at $1,000 a year) applies. Penalties cannot exceed 100 percent of the tax due — cold comfort on a short-year bill you never saw coming.

What to Do: Checklists for Each Situation​

If you are starting an Oregon business mid-year:

• Calendar the registration trigger: within 30 days of hitting $750,000 in Oregon commercial activity. The full threshold applies to your first short year — no proration.
• Track Oregon commercial activity separately from day one, not total revenue. Sourcing mistakes are the most common CAT error for new filers.
• If you expect $5,000 or more in short-year liability, set up the compressed estimated-payment schedule immediately. A business opening in October with a December year-end has a sub-three-month period and pays 100 percent with the return — but only if it remembered to forecast.
• File the short-period return by the 15th day of the fifth month after your stub year ends, then fall into the normal annual rhythm.

If you are closing or selling mid-year:

• Your final short period ends the day you dissolve or cease business — pin that date down, because the filing deadline and the estimated-payment schedule key off it.
• The full thresholds apply to your final return, but you still must file if you exceed them. Dissolving the entity does not dissolve the CAT account; file the final return and confirm the balance is zero.
• If you are being acquired into a unitary group rather than shutting down, compute both candidate due dates and file by the earlier one.
• Keep paying estimates through the closing date if the $5,000 forecast says so. The most expensive mistake in a wind-down is assuming a dying business owes nothing while its final stub still clears the tax threshold.

If you are changing your accounting period or swapping a designated entity:

• This is the case proration was written for. Count the days in the stub, divide by 365 to four decimals, and apply the factor to all three thresholds plus the $500,000 labor cap.
• Recompute exclusions and the subtraction on short-period activity only. Prior-year full-year workpapers do not carry over.
• Align your estimated payments to the compressed table above, and remember the balance-owed-with-return rule ignores extensions.

Keep Partial-Year Books That Can Survive a Short-Year Return​

Every number in this article — the day count, the prorated thresholds, the short-period-only subtraction — depends on books that can draw a clean line mid-year. That means recording Oregon commercial activity with transaction dates you trust, tagging cost inputs and labor costs to the period whose receipts they supported, and keeping the federal COGS method documented so the cost-inputs figure ties out.

Plain-text accounting makes this kind of period surgery painless: dated transactions, version-controlled history, and queries that slice activity by any date range without rebuilding the ledger. If your books already work that way, a short-period CAT return is arithmetic. If they do not, the amended rule is a good reason to start.

Keep Your Oregon Books Audit-Ready From Day One​

Whether you are opening your doors in October or closing them in June, clean period-by-period records are what stand between you and a short-year CAT surprise. Beancount.io offers plain-text accounting that is transparent, version-controlled, and AI-ready — every transaction dated and traceable, so slicing a stub year out of your books takes minutes. Get started for free and see why developers and finance professionals are switching to plain-text accounting.