How can I extend my longstay OA visa for another year before it expires in August?

Okay I think what you're really asking is how to get another year ENTRY STAMP out of a currently valid OA visa that expires in August 2022,

To do that you would exit/re-enter the country on or before the date your visa sticker expires.

At the time you enter the country you would need to meet the entry requirements in place for everyone to get back in. AND

you would need to have a new year of the health insurance as you'll get stamped in only until the insurance expires.