Can I stay in Thailand on a visitor visa while my O-A visa is being processed?

𝗪𝗵𝗮𝘁 𝗶𝗳 𝘄𝗲 𝗮𝗿𝗿𝗶𝘃𝗲 𝗶𝗻 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱 𝗼𝗻 𝗿𝗲𝗴𝘂𝗹𝗮𝗿 𝘃𝗶𝘀𝗶𝘁𝗼𝗿 𝘃𝗶𝘀𝗮, 𝘄𝗵𝗶𝗹𝗲 𝗼𝘂𝗿 𝗢-𝗔 𝘃𝗶𝘀𝗮 𝗶𝘀 𝘀𝘁𝗶𝗹𝗹 𝗯𝗲𝗶𝗻𝗴 𝗽𝗿𝗼𝗰𝗲𝘀𝘀𝗲𝗱?

There is no such thing as a 'regular visitor visa',

If you applied for an OA visa and wanted to come to thailand before it was approved you COULD enter on the free entry stamp (visa exempt entry) for 30 days

𝗜𝘀 𝘁𝗵𝗲𝗿𝗲 𝗮𝗻𝘆 𝗽𝗿𝗼𝗵𝗶𝗯𝗶𝘁𝗶𝗼𝗻 𝗼𝗿 𝗶𝗹𝗹𝗲𝗴𝗮𝗹𝗶𝘁𝘆 𝗶𝗻 𝘀𝘁𝗮𝘆𝗶𝗻𝗴 𝗶𝗻 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱 𝗹𝗲𝗴𝗮𝗹𝗹𝘆, 𝘄𝗵𝗶𝗹𝗲 𝗯𝗮𝗰𝗸 𝗵𝗼𝗺𝗲 𝗶𝗻 𝗨𝗦𝗔, 𝘁𝗵𝗲 𝗧𝗵𝗮𝗶 𝗘𝗺𝗯𝗮𝘀𝘀𝘆 𝗶𝘀 𝗽𝗿𝗼𝗰𝗲𝘀𝘀𝗶𝗻𝗴 𝗼𝘂𝗿 “𝗢-𝗔” 𝘃𝗶𝘀𝗮 ?

Nope, people apply for the eVisas all the time which aren't approved before they fly here and enter on visa exempt entries

𝗺𝘂𝘀𝘁 𝘄𝗲 𝗿𝗲𝘁𝘂𝗿𝗻 𝘁𝗼 𝗨𝗦𝗔 𝗯𝗲𝗳𝗼𝗿𝗲 𝗿𝗲𝘁𝘂𝗿𝗻𝗶𝗻𝗴 𝘁𝗼 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱 𝗼𝗿 𝗰𝗮𝗻 𝘄𝗲 𝗷𝘂𝘀𝘁 𝗴𝗼 𝘁𝗼 𝗮 𝗻𝗲𝗶𝗴𝗵𝗯𝗼𝗿𝗶𝗻𝗴 𝗰𝗼𝘂𝗻𝘁𝘆?

There is no requirement you come from the US to activate it

You do have to exit/re-enter thailand to activate a visa (it can't be done inside the country)

So if you're here on a visa exempt entry, once the OA visa is approved you'd print that PDF they email you then bounce out-n-back to activate the visa (also have proof of your mandatory year insurance policy when you go to stamp back in on the OA)