Can I leave Thailand while waiting for my Non-Immigrant O-A (retirement) Visa?

Your question doesn't make sense because you can only apply for a non-OA visa in your home country through a Thai embassy.

If you entered with a 90 day non-O then the only thing you can apply for while on that is a 1-year extension. Not for a visa, an extension. Not a non-O or a non-OA, an extension.

When you apply for a retirement extension there is not normally any waiting period. The extension is granted same day or next day.

IF for some reason the immigration office gives you an under consideration period, they will stamp your passport with a date to return. You must return on that date to get your full stamp. You can leave before then as long as you purchase a re-entry permit and as long as you return by that date. If you're not back, then you lose it and have to start over from scratch.

Also I'd be careful about making moves like that in recent events Thailand has been strickening its immigration laws and also stuff related to banking but if you're coming here just to enjoy your time and maybe see the rest of Southeast Asia along with it I hope you have safe travels and bless you

Non-OA can only be applied for in your home country. Assuming you're on your initial Non-O, yes you can leave the country but you need to buy a reentry permit.