Oklahoma (OK)
April 15, 2026
October 15, 2026
⚠ Conditional (90% Rule)
OK — OklahomaOklahoma Personal Income Tax Extension — What You Need to Do
If you have a federal extension (Form 4868) and owe no Oklahoma tax, your OK deadline is automatically extended to October 15, 2026 — no separate OK form needed. If you owe OK tax, you’ll need Form 504-I. Either way, at least 90% of your total Oklahoma tax liability must be paid by April 15, 2026, or a penalty applies regardless of which extension path you use.
Oklahoma Extension Checklist — 2026
Two things to handle. Whether you need Form 504-I depends on whether you owe Oklahoma tax — but the 90%-paid-by-April-15 rule applies either way.
April 15, 2026
Pay At Least 90% of OK Tax Due (File Form 504-I If Needed)
Check with Form 504-I Oklahoma Tax Commission
PO Box 26890
Oklahoma City, OK 73126-0890
An extension doesn’t extend the time to pay — interest still accrues.
October 15, 2026
File Federal + Oklahoma Return
File your federal return now and add your Oklahoma state return — Form 511 for residents (511-NR for nonresidents/part-year) — in the same session. Your Oklahoma extended deadline is October 15.
Secure eFile · Step-by-step guidance · 24/7 support
Oklahoma Late Filing & Payment Penalties — What It Costs to Wait
Oklahoma’s penalty for underpaying is a flat one-time charge, not a monthly one — but it applies regardless of how you extended.
| Charge Type | Rate | Cap | Trigger |
|---|---|---|---|
| Delinquent Penalty | 5% flat, one-time | 5% | Less than 90% of total OK tax liability paid by April 15, 2026 |
| Interest | 1.25% per month (15% per year) | No cap | Accrues from April 15 on any unpaid balance |
| Estimated-Tax Underpayment Interest | 20% per year | No cap | Applies specifically to underpaid estimated tax installments |
Oklahoma State Extension Details
| Extension Type | Conditional — Automatic w/ Federal Only If No OK Tax Owed |
| Original Filing Deadline | April 15, 2026 |
| Extended Filing Deadline | October 15, 2026 |
| Extension Period | 6 months (federal-aligned) |
| Extension Form | Form 504-I — only required if OK tax is owed |
| Return Form | Form 511 (residents); 511-NR (nonresidents/part-year) |
| Federal Extension Required? | Not required, but a federal extension automatically covers OK through October 15 if no OK tax is owed |
| Payment Deadline | 90% of total OK liability must be paid by April 15, 2026, regardless of extension method |
| Online Payment Portal | OkTAP |
| Extension/511-V Payment Mailing Address | Oklahoma Tax Commission, PO Box 26890, Oklahoma City, OK 73126-0890 |
| Return Mailing Address (refund or balance due) | Oklahoma Tax Commission, PO Box 26800, Oklahoma City, OK 73126-0800 |
| State Agency | Oklahoma Tax Commission |
| Delinquent Penalty & Interest | 5% flat delinquent penalty if under 90% paid; 1.25%/month (15%/year) interest; 20%/year interest on underpaid estimated tax |
Oklahoma Tax Extension — Frequently Asked Questions
Do I need to file Form 504-I?
Only if you owe Oklahoma tax. If a federal extension is all you need and you owe nothing, no OK form is required — just check the extension box on your return.
What is the 90% rule?
At least 90% of your total Oklahoma tax liability must be paid by April 15, regardless of whether you’re covered automatically by a federal extension or filed Form 504-I. Falling short triggers a flat 5% penalty on the shortfall.
Is the 5% penalty charged monthly?
No — it’s a flat, one-time 5% penalty on the underpaid amount, not a recurring monthly charge. Interest still accrues separately on any unpaid balance.
What if I’m due a refund?
If you’re due a refund, no extension form or payment is required. Filing sooner still gets your refund to you faster.
Which Oklahoma return form do I need?
Form 511 for full-year residents. Nonresidents and part-year residents use Form 511-NR.
Where do I mail my return versus my extension payment?
Extension and 504-I payments go to PO Box 26890. Returns — whether refund or balance due — go to PO Box 26800.
Ready to File? Start Federal + Oklahoma
File your federal return and add Oklahoma state in the same session. October 15 extended deadline — secure, fast, with live support.