Can I leave Thailand for a week and still secure my one-year extension on a non-immigrant O visa?

Hopefully you have had 800k baht parked up in your Thai bank at least 2 months before the date you apply for your 1 year extension.

You can't wait until as after May 19th. You MUST get the one year extension before May 19th.

Did you come in on the 90 day or get it here?

This part of your post doesn't make sense to me 𝗜'𝗺 𝗶𝗻𝘁𝗲𝗻𝗱𝗶𝗻𝗴 𝘁𝗼 𝗴𝗲𝘁 𝗮 𝟭 𝘆𝗲𝗮𝗿 𝗲𝘅𝘁𝗲𝗻𝘀𝗶𝗼𝗻 𝗮𝗳𝘁𝗲𝗿𝘄𝗮𝗿𝗱𝘀. 🙁 How would you get an extension AFTER?

You can apply for a year extension at most offices when you have 30 days or less left on your current stamp and at some offices when you have 45 days left on your current stamp. You don't lose any time applying early they add the year on to your current expiration date

Obviously you'd have to get the year extension (and buy a re-entry permit) BEFORE you leave to go to the US

BUT

No issue at all getting your yearly extension and buying a re-entry permit then leaving the country