Does my health insurance need to be valid for a full year for my O-A visa extension in Thailand?

You need to have the required health insurance coverage from one of the 15 approved insurance carriers for the O-A policies.

You need to be covered when you apply for the year extension and will get the extension for as long as the insurance is good for (up to a year).

SO if you can get the policy renewed (pay for another year which starts when this one runs out) you will get a year extension, Otherwise you just get an extension until June 2021 and then go apply for a new year extension again when that runs out