OSU Garden Planning Guide
I've linked a copy of the Oklahoma State University Garden Planning Guide below.
On one page, you'll find a listing of cool-season crops, recommended planting dates, spacing, etc. For almost all cool-season crops, the recommended planting dates fall between February 15th and March 10th. If you want to start transplants to transplant into the ground on/near those recommended planting dates, count back however many weeks you need to raise seedlings to transplant size and start your seedlings indoors about that many weeks before the recommended planting date.
On the following page, you'll find the same data for warm-season crops.
When fall garden planning/planting time comes around, there's a separate fact sheet for it.
For those of you who are new to using this guide, the earliest date given is for far southern OK and the latest gate given is for far northern OK. If you are in the middle of the state, choose a date in the middle of the recommended range of dates.
Here is a link that might be useful: OSU Garden Planning Guide