Here is the best advise I can give on the Ostwind:
First, forget using the sight. Use the tracers. Hit the F8 key and pan up all the way out of the gun sight. Again, watch the tracers and walk them in.
Second, be sure and lead. Odds are you're not leading enough. If your tracers are intersecting and you're not getting a hit sprite, lead some more. Use the training arena and have a squaddie help you walk in the hits.
Third, the learning curve is never ending. I have the most luck using the Ostwind vs level bombers and medium size aircraft (A20's, etc). It is very rare, but when you connect the target usually loses wings, get cuts in half, or goes "puuf". The range is much further than an Wirblewind, and against a fighter only a single hit is needed. Once in a while I'll see a large bomber survive a hit but very rarely.