Quote from TheAdubbz
As per the message on the first time you start the game, you need to set the level-type to BIOMESOP in server.properties for our biomes to generate
I actually have the same problem as ClockworkHearts. I use both BOP and Highlands, but didn't see any ID clashes. I've triple-checked all settings, the config files, etc etc and still we only get vanilla. There are no errors to indicate that BOP wouldn't work, and it loads properly. I'm using a remote hosting service that allows Forge, but I can't imagine that's the problem (though it may be - when I hosted an SMP server on my own personal computer, BOP worked fine). I feel like there's something simple I'm missing, but I'm not sure what.