If the Ravens started the season today, they would play Kelechi Osemele at left offensive tackle, and that would be a problem.
Osemele, who started at right tackle and left guard last season, could be a left tackle down the road, but he has to improve his foot speed during this offseason. He struggled with speed rushers on the outside as a right tackle and it would be worse for him on the left side if there isn't significant improvement.
Osemele is a tremendous talent. He is athletic, has good bend and punch and the ideal torso to be a left tackle. But to face the other team's premier pass rusher you have to have "sweet feet" -- and Osemele isn't there yet.
It's good that the Ravens pointed to Osemele as the possible left tackle, which means they have probably given up on the experiment of moving Michael Oher to play the position. Oher will likely be only used at left tackle in emergency situations.