How long can I stay outside Thailand on an O/OA retirement visa without losing it?

You only have to renew your visa in time.

As long as you return before the visa expires ,if your in your first year remember you’ll be stamped in for another year providing you have a years worth of of required insurance

I had to go to Australia and ended up staying 8 months , what I did was renew the Visa early with reentry permit , and pretty much left immediately , then got back in with 3 odd months to spare and renewed .

So , you should not have any problem .

There is no rule on maximum number of days you can be out of Thailand. Technically, you just need to be in the province on the day you can apply for an extension, which means the last day of your current stamp. But practically you may need a few more days to prepare documents for application.

For example, it wouldn't be a problem to leave Thailand immediately after you get a 1-year extension and re-entry stamp and be back in the last 5 days to prepare and apply for the next extension.