OBP measures Contact(i.e. singles hitters) and patience (Walks). using just OBP to compare skews things in favor of Contact Hitters that take a lot of Walks (i.e. Nick Johnson), and makes for an uneven comparison when comparing a Power Hitter with a Leadoff-type hitter.
Slugging measures Power (extra base hits). using just Slugging to compare skews things in favor of big Mashers that don't take a lot of walks. and again, makes for an uneven comparions.
OPS is the best of both worlds. add OBP and Slugging together, and you get a more complete and even comparison of any hitter.
at least that is how I understand it.