Does my girlfriend need to report my stay in Thailand before I arrive if I hold a Non-immigrant OA visa?

You definitely do not have to go to immigration - your arrival has to be reported (TM30) by the homeowner within 24 hours - I live in my wife's house she does it for me, every time I come and go. The house owner needs to register with immigration first( can be done online and takes 10 minutes). If you are not the owner you need the Blue book and ID card of the homeowner and you or your girlfriend can register for them. Once registered you logon to the site and it literally takes them 5 minutes to log on and record your arrival. You can also print the TM30 form if you need it later for your 90 day report.

Does she own the house or rent? Go to immigration together when you arrive. She will need copies of her Thai id, copy of house registration, if renting you need copy of the rental agreement. I think that’s all.

I was in the same situation.

Are you renting for 3 months?

You could have scored better than that pretty boy

Reporting in is required and can be done online by the homeowner.

It will help with language barrier

No, but immediately when you arrive.

You and your girlfriend should visit Immigration within 24 hours of your arrival at her home, and register you in the accomodation in the TM30 system of Immigration. She could do that online, but before she is able to register you TM30 in her home online, she would need to register her home in the TM30 system, using her blue housebook adress and number. That done, she will get issued a Login-ID and a password, and only after that she can access the TM30 system and register you

I think you’ll need to do a 90 day report as well but I’d check on that.

Not before but within 24 hours of your arrival she needs to file a TM30 form saying you are living there. She can do it online