This is the Xixo Yogg Druid Deck Old Gods format he used for recent 
tournament play. It’s a typical build that most players are using on the ladder and is one of the strongest current decks in this META.
Onyxia is a deck choice over Cenarius due to the synergy with Power of the Wild and the ability for Onyxia to get you back in the game from an empty board. It’s probably a slightly better choice and more consistent overall. But if you don’t have Onyxia, then put Cenarius in.