Oklahoma adds 30 days to every federal business deadline, so its corporate and pass-through dates all shift later than the federal calendar. Getting the pass-through date right — April, not March — is the key correction, and Oklahoma offers a low elective PTE rate.

Oklahoma Tax Deadlines at a Glance (2026)

Oklahoma computes every business return as 30 days after the federal due date. Because the federal S-corp/partnership deadline is March 16, 2026, the Oklahoma 512-S and 514 are due April 15, 2026 — not March 16 — extending to roughly October 15. The C-corporation return lands on May 15 (30 days after the federal April 15), extending to November 16. Individuals filing electronically get an extra five days, to April 20.

Rates and the Elective PTE Tax

Oklahoma’s top individual income tax rate is 4.75% for the 2025 tax year (dropping to 4.5% for 2026), and the corporate income tax rate is a flat 4.0% (reduced from 6% in 2022). Oklahoma’s elective pass-through entity tax — under the Pass-Through Entity Tax Equity Act of 2019 — is levied at just 4%, elected via Form 586 and reported on Form 587-PTE, one of the lower PTE rates nationally.

Estimates, Extensions, and Penalties

Individual estimated payments (OW-8-ES) are due April 15, June 15, September 15, 2026, and January 15, 2027. A federal extension with no Oklahoma liability automatically extends the Oklahoma return. The delinquent-payment penalty is 5% (waived if at least 90% of tax is paid by the original date), and interest accrues at 1.25% per month.

CPA Action Steps

• File 512-S and 514 pass-through returns in April (30 days after federal), not March

• Use May 15 / November 16 for the C-corp return

• Apply the 4.75% top individual (2025) / 4.0% corporate rates

• Consider the low 4% PTE election

Verify current rates and dates against the Oklahoma Tax Commission (oklahoma.gov/tax) before filing.