Actually, you will pay the sales tax to the dealer when you purchase it (he will forward this to his state oklahoma to prove he is collecting since he is a business). Then in texas, you will show proof that you paid this sales tax out of state and receive credit for that ammount and if the sales tax is higher in texas, then you will pay the difference to the state of texas. The title fees will still apply of course. Remember to bring your bill of sale w/ you to the Texas office. Do not allow Oklahoma to charge you titleing fees, only the sales tax.
good luck.
Petra
(p.s. I used to work for the county (nueces, harris) for 8 years doing the registration thing.