I have a 1099-R from military retirement. I am an Oklahoma resident and it says you can exclude 75% of retirement. But 1099R box 1 and 2a are same amount. Is this right?

Box 2a is for the federal return, not the state return.

Depending on the state, you will be asked about the source of this 1099-R income. Some states ask right after you enter the 1099-R on the federal return, other states ask as part of the state interview what the source of the income is.

Oklahoma appears to do this in the state interview. On the screen in the Oklahoma interview headed "Pension Income", answer "yes". On the next screen(s), you will be asked about each 1099-R by the name that you entered on the federal return. Indicate that your Military retirement 1099-R was military, and TurboTax will handle the exclusion.