You can use the corner of an old clipped up credit card to run down the groove in the bar to clean it out. I do this anytime I flip my bar. I also blow out the sprocket cavity, and the oil holes. As mentioned, the oil holes can also plug up and impede flow, but if it's just sawdust, it won't make too much difference (hard to believe, but true). If it's a chip or a solid piece of wood, then you definitely want to clean out the holes.
That said, recent model Stihls do not visibly oil like older saws, and it's misleading. You might see a light mist at the bar tip, but the saws will not sling oil like the old days. The only way to tell if they are oiling right is to confirm that you go through about 3/4 tank of oil per 1 tank of gas. On the flip side, if the chain binds and the saw struggles, then it's probably oiling poorly.
By experience, I can tell if the saw is oiling by the sound the chain makes as it runs along the bar. In fact, if you ever put a new bar/chain on a saw, listen to it and you'll hear the proper sound once the oil starts flowing all the way around the bar. It goes from making a metal-on-metal racket to something more like metal-on-butter...