Leading off on Mark's comment wrt how much space O scale takes, my train room is 11'8" X 12'8" and one simple around-the-walls loop is what fits nicely. Granted that I am limited by a 42" high knee-wall which pretty much precludes a twice-around up-and-over track-plan.
Looking West, leaving room for a futon guest-bed means a single track on a narrow shelf along the North wall:
Facing South. Even modest O scale structures use a lot of real estate:
Situating industries in corners takes advantage of available space. The East End:
The West End:
Doorways are alway a constraint. The Northeast corner:
There is just nicely room for a single track along the east wall. Again, O scale is BIG. But as with other folks here, I find the visual impact worth the sacrifice. I don't have to squint while setting-out or picking-up a car on an industry siding :