What do OLB's do in 3-4 system?

In a 3-4 defense, the two Outside Linebackers (OLBs) often line up closer to the line of scrimmage than the Inside Linebackers (ILBs).

One of the 3-4 OLBs, the Jack, is usually the primary pass rusher. Depending on the scheme, the Jack (also known as the Joker, Buck, or Elephant) can line up on either side of the defensive formation. The Jack Linebacker must be an excellent pass rusher, and has to be able to beat both stronger right tackles and rangier left tackles off the edge of the formation.

The other 3-4 OLB, the Sam, does not have a specific designation. As with the Sam Linebacker in a 4-3 scheme, the Sam Linebacker in the 3-4 must be able to cover, blitz, and play the run.

In most cases, 3-4 OLBs lead their teams in quarterback sacks.

Usually, teams that run a 3-4 defense look for college "tweeners"--defensive ends that are too small to play the position in the pros and not quite fluid enough to play outside linebacker in a 4-3 defense--as their 3-4 outside linebacker.

rush passer set edge

It depends on the level and situation. They mostly have contain responsibility. If it's a pass they either cover the flats and rotate towards the play or rush.

So when people talk about how it varies dependent on scheme what they're referring to is the kind of plays a team will run from whichever formation.

In a 4-3 the 4 linemen are generally responsible for attempting to pressure the QB while the 3 Linebackers typically blitz far less than in a 3-4, therefore they will typically have coverage duties, either playing man to man coverage against TEs or RBs or covering the flat and short middle in zones. DE's in a 4-3 will sometimes move out to cover the flats but 90% of the time or more they're attempting to get around the offensive tackle and get to the QB as quickly as possible. Against mobile (i.e. running) QBs they also have contain responsibilities to not let a QB leave the pocket and push him in to the other defenders.

In a 3-4 you will typically see a lot of blitzes, but most OLBs won't rush the passer on every pass attempt. With a 3-4 a coordinator will occasionally drop all 4 LBs into coverage and rely on just the three linemen to generate pressure, or use delayed blitzes where the linebackers wait a moment to see how the line will block the three offensive linemen and then rush the gaps left in the line. Teams will also frequently use different combinations of blitzers to confuse the offensive line, one play it'll be just the ROLB, the next the LOLB and the WILB, etc. Generally speaking, your elite pass rushers are going to come from the OLB position in a 3-4 because the linemen's primary job in that formation is to occupy two gaps against the run and eat up blockers to let the LBs move freely, and ILBs are typically more responsible for run stuffing and reading the play, not getting that quick penetration before a play develops.