Omega is a hypothetical super-intelligent being used in philosophical problems. Omega is most commonly used as the predictor in Newcomb's problem. Sometimes Omega is taken as a perfect predictor, while other times it is an almost perfect predictor. In some thought experiments, Omega is also taken to be super-powerful.
Omega can be seen as analogous to Laplace's demon, or as the closest approximation to the Demon capable of existing in our universe. Including a perfect predictor in a problem can lead to subtle issues such particular counterfactuals being undefined. See the article on Perfect Predictors for discussion of the technicalities and discussions about the validity of perfect predictors.