Outside linebackers are becoming freakishly athletic for their size at the collegiate level. Much like the way defensive linemen are recruited, schools choose an outside linebacker depending on whether they primarily play a 4-3 or 3-4. Ideally, the linebacker will be able to play in both schemes.
In 4-3 defenses, the outside linebacker must be a good athlete that can play in space. They have considerable cover responsibilities and should be able to drop into zones and cover running backs out of the backfield. They must have good hips and flexibility, as well as the speed and range to chase down plays.
Most OLBs do not spend a lot of time rushing the quarterback, but the ability to pass rush is a big bonus. They must also be solid open field tacklers and have the ability to get off blocks and flow to the play.
That being said, there are few outside linebackers at the Division I-A level who are complete players. Most enter college adequate against the run or pass, but not great at either. Finding an outside linebacker that can do it all is difficult because these players are often underdeveloped and drastically undersized.
In the 3-4 defense, the outside linebackers become the primary pass rushers. They have very little pass cover responsibility and usually are "up field" guys that beat the offensive linemen with their quickness. Most premier outside linebackers at the high school level possess great speed and a nose for the ball, but will struggle playing the run because of a lack of bulk and overall strength.
Many high school safeties will also become excellent outside linebackers as they mature and outgrow the safety position, because of the lack of overall speed and quickness needed to play in the secondary at the Division I-A level.