Can I hold both a retirement visa and a work permit in Thailand?

Okay, almost NO ministry of labour will issue a work permit when someone is on a yearly extension of stay based on being over 50 (retirement). I know people who have had them and this year they were forced by the MOL to switch to a different type of extension OR lose their work permit.

What you would do is change your extension from one based on "retirement" to one based on working

OR

If the company you will be working for can't do that you'd take documentation from them to a thai consulate in another country and get a 90 day single entry Non-B <- the B means business visa so you can hold a work permit..

My original O-A has printed on it “employment prohibited”. My understanding is that also applies to my yearly extension .

You cannot be on a retirement visa and then work legally in Thailand. I know someone who got busted for that and he wasn't even "working", just on suspicion of doing so was enough to usher in a bunch of immigration police officers to raid the place he was at and to secure the passports of everyone there to check for overstays, illegal work, etc. He had to pay a large fine at the very least.

Your employer is not going to provide you a visa, only Immigration Office or a Thai Consulate can provide a visa. Your employer has to apply for the Work Permit at the Ministry of Labor and you have to change from your "retirement visa" to a Non Immigrant B visa.