The wiki here doesn’t have entries for OK or TX. From moped2.org, it appears that you can ride every road except "high speed highways" or of course interstates, in OK. If the road has mailboxes, driveways, and side roads connecting right to the road it is probably not a high speed highway.
Thats a pretty vague term for Texas. In TX, some narrow, 2 lane state and US highways with really no shoulder to speak of are 70MPH. I know, I've driven some. Makes no sense at all..
Along a US highway leading out of TX and into OK, I think the OK speed limit on the same highway went down to 60-65 and down as low as 45-50 on some of the hard curves they put in between the surrounding farm land.