There is videos about hop up assembly on youtube.
Really? GEE, Thanks Tips. No links?
Here is something more useful and a few links, hope it helps man.
The o-ring might actually be a replacement of your bore o-ring. mine came with one also.
I could be wrong but i did that with mine and it works.
Airsoft GI asembly
YouTube - Airsoft GI 101 - M4/M16 Hop Up Assembly Instruction
Assemble and disassemble
YouTube - Airsoft Inner Barrel and Hop Up Assembly and Disassembly