Hard to say "This is how you hit people with an Osti" when there are so many variables. Long shots of 2k or more I use the "shotgun" method. I lead them and try to guess where they will be and fire alot of shots. Close shots depend on the AOA, speed and even plane type. Practice, practice, practice, practice, practice, practice, practice and you'll get better. I have killed planes as far as 4k away....don't happen offen but it can be done. If your ever tearing down a town with an osti zoom all the way in a watch the rounds...talk about all over the place. They are very inaccurate and as the round goes further the more off target it gets. So some will say...let them get closer before you fire. If you do that either your about to have an egg dropped on you or you have no turret. As far as survival if there are multiple friendly flaks out I am usually the 2-3 one to start firing. Usually you mark yourself as a target before the baddy even gets an icon on you. If there are low buffs coming in I don't fire till I am sure of thier flight path. One of two things happen either I start moving or I start firing.
Also do you use the cursor, page up method? It really helps in field of vision. PM me or look me up in the game and I'll explain it to you.