ESPN NFL Nation reporter Paul Gutierrez explains what the re-signing of Donald Penn means for Kelechi Osemele and the rest of the Raiders' line. (2:25)

There's often no quicker way for teams to repair ailing offensive lines than by dipping into the free-agent market and signing a proven commodity rather than rolling the dice in the draft.

That's exactly what the Raiders have done. After finishing the 2015 season as the sixth-ranked line by Pro Football Focus, the Raiders went out and locked up the top free-agent offensive lineman in Kelechi Osemele and retained their solid left tackle, Donald Penn. Now, on paper, Oakland has the league's best unit.

Here are the five best offensive lines in the NFL:

Average starter rating: 82.7

The Raiders were already in good shape last season, but the addition of Kelechi Osemele moves them to the top spot in the league. Retaining left tackle Donald Penn means Osemele can stay inside at guard, his best position. Oakland is now the only unit in the league to have every starter rate above a PFF rating of 79.0. Osemele has been in the top five of PFF's run-blocking grades in each of the past two seasons.