Blacke and Roy defend just fine, and Oden is one of the quickest 7 footers in the league.
No they don't and no he isn't. He's more mobile and quicker than last season (when he was really slow) - he's improved to Roy Hibbert's territory. He's still slow moving his feet and reading/reacting to plays.
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This season the Blazers are defending the ballscreen differently than they did the last one (where they'd end up switching on big/ball screens way too much), precisely to minimize Oden's impact. While they still have Aldridge showing up, Oden drops down off the screener and zones to deny the penetration to the ball-handler or the roll. This is probably the best possible solution if you have a player like him, but the downside can be big: guards with good shooting off-the-dribble will have a moment to shoot if they run the play well (until the defender closes on him; if Oden is close enough to contest the shot, they'll just go through him after turning the corner); the screener can pop out a couple of steps, receive the pass and shoot over a Greg Oden recovering upwards (they're more vulnerable to the pick'n'pop than exactly to the pick'n'roll, I think). In a play-off series this will constitute a very important factor for some teams that have the tools to take advantage of it (namely ones with good jump-shooting bigs). The Lakers and the Jazz don't run the ballscreen very often except late in possessions, but they can kill the Blazers with this play.