I just started playing so take this as far from gospel, but so far I would suggest a happy medium between what your scouts think about a player and what OSA thinks. This seems to accomplish two things:
1) Makes it more likely your scouts ratings are legit.
2) Maybe allows you to sneak a guy your scout says is great until your next pick. If OSA doesn't like him much, you've got a decent chance of his sticking around a bit.
Between that and keeping a little bit of an eye on his stats and maybe you get somewhere. Also depending on how you have your potential ratings set, you should probably steer clear of any middle relievers unless they have an expected role as a starter. Seems to me accumulating relievers through other means than the draft is easy and they tend to fall down the draft board anyway. What's nice about that is that it kind of works the same as real life.
I mention all of these things because they are all mistakes I have already made.
Nothing like a little bit of trial and a whole lot of error.