How can I renew my OA visa before leaving Thailand if it is due on July 21?

I have placed the TOPIC of Conversion of the OA based visa to Non O Visa as retired on your post...

CLICK on it and all posts about how changing the OA to Non O is done...

Ask your local Thai Immigration office... As many will allow you to get the one year extension of stay up to 45 days prior to its expiration date. And even a little more time in advance with PROOF of your needing to depart Thailand. Solely at the discretion of the Thai Immigration office.

IF you do get the OA extension of stay for one year before you leave Thailand, be sure to buy a re-entry permit to re-enter Thailand with.

Be sure to have the OA health insurance required for your one year extension of stay. The new amount of insurance required is US$100,000 or 3 MILLION Thai baht.

And Yes, if you chose not to extend it before you leave, you can enter Thailand Visa EXEMPT and get either a 45 or 30 day stamp on arrival... Plus one 30 day extension of stay at Thai immigration office for 1900 Thai baht.

Followed by the Non O Visa as retiree and the One year extension of stay at your local Thai immigration office.

If you're in Bangkok you can renew 45 days in advance so very close. I'd have thought if you are able to show your flight out then a few days earlier than that would be acceptable.