So apparently Oklahoma doesn't require a title, registration, license plate, nothing on private use trailers. Cant imagine the Brazos tax office is going to take my word for it. Anyone ever been down this road?

Tornado bait

I bought a trailer that had California registration, just had to fill out the paperwork, get it inspected, and pay the taxes on it. Pretty simple process if the tax office people don't screw anything up which happened twice to me.

I bought a new horse trailer that was manufactured in Oklahoma. The dealer gave me a receipt and bill of sale with the price I paid for it and the MSO (manufacturer's statement of origin). I had to sign an affidavit that showed I was taking the trailer and resided in Texas and would responsible for paying the sales tax in Texas.
I took it to our county tax office, gave them that paperwork, paid the required sales tax due, paid for the license plates etc.
If you are buying a used trailer and the owner lives in Oklahoma he should still have the MSO's. He needs to give you a bill of sale noting the sales price and the MSO's.
Oklahoma residents are not required to have titles or tags if the use of the trailer is non commercial.
A lot of guys I know that live in Oklahoma and fish bass tournaments in other states pay like 10.00 and get trailer tags from OK DMV in case they are stopped by LEO's in other states.
All trailers built should have a MSO no matter where they are built…. at least that is what we have to do on our boat trailers.
Hope this helps.

my thoughts:

get a BOS or make one yourself, go have it inspected and weighed

take docs to tax office and get it titled and registered. I'm not sure what other steps you would take.

I'd cal my tax office and ask them before I went to buy the trailer and ask what they will need. Mine was very helpful when my dad gave me grandads truck as a gift. Still had the title from 1973 and granny's signature on it.

https://www.microskiff.com/threads/texas-homemade-trailer-registration-how-to.113417/

Good info here.

Thanks for all the info. It's a used trailer that came from Florida so no MSO. I'm getting a screaming deal on an offshore boat so dont want to wait as long as the paperwork for the boat checks out. Heading north here in a bit. Will call tax office when they open.

Off shore boat, screaming deal, in Oklahoma.....

Something sounds fishy here

If you run into land mines and end up needing an MSO you should be able to get what is known as a duplicate MSO from the trailer manufacturer if they are still in business. Per law it had one when it was built so there is one somewhere if it's never been titled. We do that (duplicate MSO's) for dealers on our boats and trailers often.
They will just need the VIN of the trailer.
Good luck on your new endeavor.

ETA: There should be MSO's for the boat and motor/motors as well if they were purchased but never titled. Boats over 14' and outboards over 10 HP are supposed to be titled in OK….so hopefully your seller has those titles for you.