Climate and Hardiness Zones
Writing for the Oklahoma Climatological Survey, climatologist Derek Arndt describes the climate of Oklahoma as continental, with long, hot summers and short winters with rare periods of extreme cold. (see reference 1)
Most of the state lies within hardiness zones 7b in the southeast, where the lowest winter temperature rarely drops below 5 degrees Fahrenheit, and 6a in the panhandle, where the temperatures may dip as low as -10 in the winter. (see reference 2)
What that means, in practical application, is that in most of Oklahoma, the last frost comes in early spring, and the first frost doesn't occur till sometime in October. This adds up to a long growing season of more than 200 days.
Determining what the first and last frost dates are for your area can help you figure out when you can plant your vegetables. The average frost dates in Oklahoma don't vary as wildly as they do in northern states like Idaho or Utah, but they can differ by as much as a month. According to Victory Seed Co., which propagates and sells vegetable seeds, Boise City, in the panhandle just a few miles from the Colorado border, usually has hard frosts up until mid-May, while in Tulsa, the last frost usually occurs in mid-April. (see reference 3)
(rest of information is from resource 1) You should decide what you'd like to plant and then determine how hardy those vegetables are. The Oklahoma Cooperative Extension (OCE), a division of Oklahoma State University in Stillwater, publishes a detailed garden guide categorizing different vegetables by hardiness and listing their optimum planting dates.
David Hillock and Brenda Simons, extension consumer horticulturalists for OCE, distinguish between hardy, semi-hardy, tender or very tender vegetables. "Hardy types may be planted before the last killing frost," they write. "The semi-hardy ones will be injured by a hard frost, but will grow in cool weather and not be harmed by a light frost. Tender plants are injured or may be killed by a light frost but can withstand cool weather, while the very tender are injured by cool weather."
Hardy and semi-hardy vegetables include members of the cabbage family like broccoli and cauliflower, leaf lettuce, onions and green peas. Hillock and Simons advise you to plant them between mid-February and March 10, depending on when the last frost usually takes place in your location.
Observing that same caveat, you should wait to plant tender vegetables like peppers, tomatoes and beans until sometime between April 10 and 30. Very tender plants such as melons and cucumbers can be planted between mid-April and mid-May.