I posted this in a comment before but I think its important enough to clarify the difference between playing DE in a 4-3 vs playing OLB in a 3-4 because they are different play styles most of the time and its also a reason that drafting a DE in college to play OLB in the pro's doesn't work as much as people think.
When Playing the run in a 4-3 you usually have one end that maintains outside contain (usually the worse of the two pass rushers) unless is is an obvious passing down. Now the other end is free to rush the passer almost every single play. Simon Rice was a good example of this because of his dynamic pass rushing ability he had defensive schemes built around him to allow him to rush the passer every single down which is why sometimes he let runners run right by him. The way you are suppose to overcome these short comings is the OLB in the 4-3 is now responsible for outside contain that allows the other two line backers to flow to the ball carrier.
3-4 or 5-2 same thing (most of the time)
In a hybrid 5-2 an OLB has a progression (unless the design of the play calls for a pure blitz from that backer or what it looks like Ray Horton likes to do is blitz inside backers bring some corners and drop an OLB in a zone, think Harrison in the longest Super Bowl Play ever dropping back and intercepting a Kurt Warner Touchdown. Im going to go vomit now......):
1) outside contain, you can figure this out in a half a second if the Tackle attacks you and is trying to seal the corner you attack that outside shoulder and get separation from him extending the play and make that ball carrier flow back into the Inside line backers
2) If a RB crosses your face and runs to the flats you run with him, this is for any passes to the flats or screens, on tv this is why you see good OLB starting to pass rush then they stop and run with the back
3) Rush the passer, get to him by any means necessary.
Part of the reason you draft someone who plays OLB in a 3-4 from college is it takes a year or two to adjust to these differences in playing style. Also when you draft a 4-3 end it could of been a complete farce because they let one player rush the passer every single play. Im not saying it cant work but it takes the right DE to become an OLB.