i just recently finished my first mold for an OM. after being unhappy with a first attempt with birch plywood (not the baltic birch, just regular old hardware store version, which isnt all that cheap by the way, about $40 for a 4x8 sheet), i decided to go with the less expensive 3/4" MDF. i didnt feel like the plywood was flat enough, but quite possible i was overthinking things. anyways, im making an OM and i made my mold 2 1/4" thick (3 layers of MDF). i believe in robbie o'brien's videos his mold is only 2 layers thick, although i could be wrong. you want it thick enough obviously to support the sides, but not so thick it makes installing the linings a pain in the rear.
to actually make the mold, i just made a template and used a bandsaw and a pattern cutting bit to shape the individual pieces, then glued them together. it helps to use some sort of nail or something to keep the pieces from sliding around until the glue sets up. then you have a small amount of sanding to do just get things right. this is obviously the abridged version and i think this will better explain. http://www.luthiersforum.com/forum/forum_posts.asp?TID=10884 &KW=tutorial#forumTop
the only thing i will say is that i used the "small" de-sta-co clamps at either end to hold the thing together, and while they are convenient, with the spreaders pushed against the sides, the mold starts to separate a bit at the top and bottom. you probably dont need much pressure pushing out to get things snug though. so you might want to consider what kind of fastener you use to hold the two halves together.
good luck and have fun. i think building this sort of stuff yourself is a great part of building. even though it has been a bit frustrating and i cant wait to actually get along to building a real guitar. 