How to Fill Out and File Oklahoma Form 504-C: Deadlines and Payment Rules

Oklahoma Form 504-C is the Oklahoma Tax Commission’s application to extend the filing deadline for a corporate, partnership, or fiduciary income tax return. It buys time to file the return, not time to pay. To be valid, the form must be submitted by the original due date and accompanied by enough payment to cover at least 90 percent of the entity’s Oklahoma tax liability for the year.

When You Need Form 504-C and When You Don’t

Oklahoma honors an approved federal extension as a valid state extension, but only when no additional Oklahoma income tax is owed. If estimated payments and credits already cover the state liability, you can skip Form 504-C and simply attach a copy of the federal extension to the Oklahoma return when you file it.¹

Form 504-C becomes mandatory in three situations:

The entity expects to owe additional Oklahoma income tax beyond what has already been paid.
No federal extension was requested, so Oklahoma has nothing to honor.
The federal extension period is not long enough and more time is needed up to Oklahoma’s maximum.

Filing a federal extension does not stop Oklahoma penalties from accruing on an unpaid state balance. If tax is owed, Form 504-C is not optional.¹

Who Uses the Form

Form 504-C covers C-corporations, S-corporations, partnerships, and fiduciary entities such as trusts and estates.¹

One exception matters. S-corporations and partnerships that have elected to pay Oklahoma income tax at the entity level under the pass-through entity tax use Form 504-PTE instead. If your entity made that election, Form 504-C is the wrong document.¹

Filing Deadline and Length of the Extension

The form must be postmarked on or before the original due date of the return, or before the expiration of an automatic federal extension if the entity is relying on one. For a calendar-year C-corporation, the Oklahoma return is due May 15. For a calendar-year partnership or S-corporation filing Form 513, the Oklahoma due date is April 15.²

The maximum extension depends on the entity type:

Corporations get up to seven months beyond the original due date. A calendar-year C-corporation with a May 15 deadline would have an extended deadline of December 15.
Partnerships and fiduciaries get up to half the accounting period, which is six months for a standard 12-month tax year. A calendar-year partnership with an April 15 deadline would have an extended deadline of October 15.

How to Fill Out the Form

Form 504-C is a single page available on the Oklahoma Tax Commission website under business tax forms. It serves as both the extension request and a payment worksheet.

Entity Information

Enter the entity’s legal name, mailing address, and Federal Employer Identification Number. Check the box for calendar year or fiscal year and fill in the applicable tax year dates.¹

Payment Lines

The lower half of the form determines whether a payment is required.

Line 1 is the estimated total Oklahoma income tax liability for the year. Enter a figure even if you expect to owe nothing; use zero in that case.
Line 2 is the total already paid for the year through estimated tax payments, prior-year credits carried forward, and other remittances.
Line 3 is Line 1 minus Line 2. If the result is positive, send that amount with the form. If payments already equal or exceed the liability, no payment is needed, but you still file the form if you lack a federal extension or need time beyond it.

An authorized officer or preparer signs and dates the form, attesting that the estimates are made in good faith.¹

The 90 Percent Payment Rule

The extension is not granted unless at least 90 percent of the entity’s total Oklahoma tax liability is paid by the original due date. If payments on Line 2 come in under that threshold, the extension is treated as invalid, the return is considered delinquent, and penalty and interest run from the original due date.¹

This is the most common way extension requests fail. When estimating Line 1, err on the high side. Underestimating by enough to drop below 90 percent voids the extension and exposes the entity to penalties on the full unpaid amount.

How to Submit the Form

Electronically Through OkTAP

Extension payments can be submitted through the Oklahoma Taxpayer Access Point at oktap.tax.ok.gov. If you pay electronically, do not also mail the paper form. Keep a copy of the completed Form 504-C and include it with the income tax return when it is filed.¹

By Mail

Mail the completed form with any payment to:

Oklahoma Tax Commission
PO Box 26890
Oklahoma City, OK 73126-0890

Write the Federal Employer Identification Number on the memo line of the check so the payment posts to the correct account. The envelope must be postmarked on or before the original filing deadline.¹

Penalties and Interest on Unpaid Tax

Two separate charges apply to balances not paid by the original due date.

A delinquent penalty of 5 percent of the total tax due is assessed when less than 90 percent of the liability is paid on time. It is a one-time charge, not a monthly accrual.³
Interest accrues at 1.25 percent per month on unpaid tax from the original due date until the balance is paid.⁴

The Tax Commission will waive the penalty if the taxpayer pays the tax and interest within 60 days of receiving a proposed assessment, or voluntarily pays upon filing an amended return. Interest is not waivable and continues to accrue regardless.³

What to Keep and What to Attach Later

The Tax Commission does not send a written approval. The extension is presumed valid if the form was timely filed and the 90 percent threshold was met. Hold on to proof of filing: the OkTAP transaction record for electronic submissions, or a certified mail receipt for paper submissions.

When the income tax return is filed later, attach a copy of Form 504-C to it. If you relied on a federal extension instead because no Oklahoma tax was owed, attach a copy of the federal extension to the Oklahoma return.¹

1
Oklahoma Tax Commission. Form 504-C – Application for Extension of Time to File an Oklahoma Income Tax Return
2
New York Codes, Rules and Regulations. Oklahoma Code 68-2368 – Persons Required to Make Returns – Income of Estates and Trusts
3
New York Codes, Rules and Regulations. Oklahoma Code 68-2375 – Payment of Tax – Delinquency – Penalties and Interest
4
Justia. Oklahoma Code 68-217 – Interest and Penalties on Delinquent Taxes – Interest on Refunds