I'm pretty sure Onaga is just the name of the Dragon King, though I'm not positive. But it's my understanding from the endings of Deadly Alliance that Reptile himself is the Dragon King. So if Onaga turns out to just be the name of the Dragon King, then yes, Reptile is Onaga.

I looked like he is judging from the DA ending, but no official story has said it yet.

I dunno I think reptile was more of a segway for the dragon king. Reptile may be dead, consumed by the energy of the dragon king.. one thing is for sure it's not reptile controling that mofo.

Does someone have to be the dragon king?

Don't flame me but this was part of the source code in MKD official site.

"meta name="Keywords" CONTENT="Midway,Mortal,Kombat,Mortal Kombat,Ed Boon,This,Baraka,Is,Dragon King,Deception,Mileena,Raiden,Scorpion,Sub-Zero,MKD,online,GameSpy,fighting,tournaments,puzzle,fatalities,character,hari kari,backgrounds,breakable,Konquest,Chess Kombat,Puzzle Kombat,supernatural,storyline,mythology,mystique"

of course reading that something is gonna catch your attention. I am not gonna say it so no one will say I said.
Hey, the game is called deception right?