A while back, I posited about how Ovechkin has a real chance to surpass Wayne Gretzky as the greatest goalscorer of all time. He is still on pace for that. He's already ahead of Gretzky in goals adjusted by era (Gretzky's era had far more goals per game overall than Ovechkin's).
That said, even if Ovechkin surpasses Gretzky in goal-scoring, he won't even come close to overtaking him as the greatest offensive player ever. In addition to his 894 goals, Gretzky also had a virtually unthinkable 1963 assists - far surpassing Ovechkin (601) and everyone else. I won't run the numbers, but Gretzky also had a far superior playoff record to Ovechkin.
If you consider assists as well as goals, it is clear that Gretzky was the greatest offensive player ever, by a large margin. He probably is also the greatest player at any position. But it's tougher to compare him to defensemen and goalies, especially given the very limited quality of hockey stats for defensive contributions.