I would start by getting a track plan together first. You really cant build effective bench work if you don't know where the tracks will be placed.
You are really trying to put a lot into a small space. Most people think about O Scale is twice as big as HO but truly you are looking at taking up a lot more than double the space. An O Scale building takes up considerably more room than an HO building. Same with all you hills and bridges and mountains too. Get a cardboard box make yourself a box the size of a normal building and check out how much room you are going to need for even a small town area.
I would recommend getting some graft paper and a ruler and kind of draw out a few sketches then start back on the bench work.
My layout is 20x15 around the walls I divided it into half town and industry and half country through the hills running. It is a tight space to get everything I want.
I think you will save some discouragement if you put a pen to paper before you put a saw to wood.
Oh and welcome to O Scale!!!
