just wondered if there is anyway to move the date of my extension from august to December by  maybe paying extra or applying 2 times in one year december is more convinient to me

No.

The only way to change the date is to start over with a new O visa.

Probably not a good idea to switch from August to December in any event, since December is high season here and your office may well be lain to siege by hordes of tourists seeking extensions then. On the other hand August is low season so you should get your extension processed rather more quickly then.

On what basis are you here?

If an unexpired one year OA Visa you can leave out/in when you choose and get one more year to that date, then apply for an extension.

I am doing this to get away from December which is painful in Phuket and I'd avoid High Season everywhere there is tourists.

I think it's already strongly implied in the way he phrased the question, that he has already entered the extension in Thailand system, meaning the properties of an O-A visa (if he started with that) are irrelevant.