sounds like you need to get trottier's 'bible'... despite controversy on many fronts, all you'll find there, complete with examples, will be acceptable to any agent/prodco...
as for your questions:
1. Should full-length film be 110 or 120 pages?
today's general industry preference is for nothing longer than 110 for drama or action scripts and under 100 for comedy... keep to those limits and you'll be ok... it's just the first look at the script that matters here, and page count is the first thing most mail scanners or readers will check...
2. Should you avoid using P.O.V., CLOSEUP, ANGLE ON, etc. Advice was that they clutter and are insulting to the director. Another book says to use them, but avoid CAMERA.
ALL camera directions should be avoided unless you're going to produce and direct your own work... it's the director's job to decide such things, not the writer's... use POV only if it's absolutely necessary to the plot... most new writers don't know how to use it properly, so study its use in good scripts by the best writers and in trottier's 'bible' before you even decide whether you need to use it or not...
3. What's O.S. stand for?
'off screen'... it means the actor is PHYSICALLY PRESENT on the SAME SET, but just not 'on camera' while speaking [such as in a closet or speaking from the next room]... it does NOT refer to just a 'voice' that is HEARD 'in the scene' while the BODY that voice belongs to is NOT present in the scene [on the set], such as in phone calls, when the voice on the other end is heard... in such situations, the proper term to use is V.O. [voice over]... see pages 139-141 in trottier’s ‘bible’ for the full story on VO vs OS…
hope this helps... for a 'tools of the trade' list, you can email me at:
[email protected]
hugs, maia