Oliver is a system of player projections developed by Brian Cartwright and used by The Hardball Times. For projecting returning Major League players, it uses a somewhat basic formula: Three years of player data with the most recent years weighted heavier, while also factoring age and regression to the mean.
In that way, Oliver is not unlike the Marcel projection system, which is known to be one of the most basic projection models. But where Oliver differs is in its method for projecting Minor League players' transitions to the Majors -- or in projecting younger players with minimal big league experience.
Whereas Marcel simply uses league averages for those players -- and other projection systems weigh trends from each Minor League level equally -- Oliver uses its own formula to assess Minor League data.
Obviously no one claims that Oliver's projections will be perfect, but it is widely regarded as one of the best predictors for players transitioning from the Minors to the Majors.