My tanong is, I know David Tennant and Georgia Moffett recently had a baby girl they named Olivia, What would her last name be? will it be Moffett her mother's surname or McDonald, her father's real surname or Tennant?

Her name is actually Olive, not Olivia (Peter Davison told the Daily Mail over the phone and they misheard and printed the wrong name, and in the oras it took for them to notice and correct it, other websites had already copied). Peter Davison's clarified that it is Olive.

No one apart from them knows what the surname is, but it's either Moffett or McDonald (not Tennant as he never changed it legally, although he's sinabi he thinks of it as being his 'real' name and doesn't identify with his birth name any more). David's so private he won't acknowledge either of them publicly, and Georgia keeps fairly private too. So unless the tabloids dig out her birth certificate, or Peter Davison reveals it, we probably won't know. Sorry.

Maybe it's oliba McDonald Moffett Tennant? xD

I'd say that because when Georgia was born, her parents decided to have her keep Peter Davison's real last name, that oliba should be ibingiay David's real last name, but I'm not so sure because David and Georgia had oliba before they got married, and I'm still confused on the whole "Real surname or psyudosurname?" ordeal that David and Peter both cause for me.

Her real name is Olive. And I would say her last name would be McDonald.

Tennant.

OK, My answer is all 3, because there is so much confusion I decided to look it up and here is what I got : oliba McDonald, Tyler Moffett, Wilfred Tennant.
I hope this was useful :)

David has actually stated that he had to legally change his surname to Tennant to meet screen actors guild rules so I wouldn't be surprised if they are all Tennants.