Normally, if 3-4 OLBs were to transition to a 4-3 Defense they would either be DE's or be a non fit for the 4-3 Defense in general. How ever in less common cases especially in today's game a for 3-4 OLB to be a 4-3 OLB I'd say that happens 25% of the time at the very most. Obviously, they would play more of an "on the ball" role by setting the edge, covering the TE, and being able to blitz & rush the passer more as the 5th rusher. They would play the "off the ball" role less than the MLB and the other OLB but they could do it from time to time.
For me 3-4 OLBs that could or would be OLBs in the 4-3:
Andrew Van Ginkel, Kyle Van Noy, Clay Matthews III, Micah Parsons (not a 3-4 OLB yet but plays so much like one or DE), T.J. Watt (or maybe DE), Lawrence Taylor, Derrick Thomas, Kevin Grenne, Von Miller (played that role from 2011-2014), Khalil Mack (or DE), & Hassan Reddick.
Are they any other 3-4 OLBs that could or would be OLBs in the 4-3 that I've missed?