All of the plastic structure kits I have seen for O gauge (or HO or N, for that matter) are simple by comparison to Tamiya and similar kits: think of the simplest model tanks kits (e.g, about ten pieces): maybe the worst is a bit more complex than that. You have a lot of windows in some buildings and so more opportunity to glue on clear plastic and mar thire finish than on tanks and rockets, but other than I'd rate that O gauge buildings easier to do than model tanks.
Wooden kits run the range from fairly-easy-but-different (way different than plastic) to nearly imossible. The nearly impossible kits are because the instructions are nearly obtuse and the kit little more than the materials you need for a scratch-build. Bar Mills and others won't be that difficult: wood is a different materiel and wood glue is a very different glue - it is just different than plastic. Try it. Wood is fun.
It is the resin kits I have found the most difficult. Not that difficult, not like some resin model ship kits (the most difficult kits I run into, period). Not hard to understand what to do, but I've encountered my share of warped parts or brittle parts that have to be heavily trimmed and don't quite fit.