When a facehugger captures a character at the begginning of the build phase, can i pay immediately to KO him, or should i wait the next turn?
You have to wait until next turn.
There's only one moment when "At the start of your Build Phase" happens. So let's say that triggers your Ovomorph to try to Hatch, which then leads to one of your Facehuggers capturing a character. It would now be too late for the Facehugger's "Burst" power to trigger.
Also, in case you're curious -- there's a great facebook group called the Vs 2PCG Collective. People are really friendly and they answer rules questions really fast. (And it's way more active than the BGG Vs page.)