I got Better Cities not long ago, did everything I was supposed to (using OBMM) but when I play the game, there's a ton of the big yellow exclamation marks.

Any ways I can fix this?

Do you have ALL the .BSA files installed? Did you install both resource packages?

There should be seven .BSAs in your Data folder:

Better Cities - Textures.bsa
Better Cities - Bravil Docks.bsa (or the one for Blood&Mud/B&M QTP3, whichever you're using)
Better Cities - Shipyard.bsa (or the one for Grumblepunk's Shipyard)
Better Cities - Meshes.bsa
Better Cities - Textures Complement.bsa
Better Cities - Sounds EV.bsa
Better Cities - Textures EV.bsa

I'm pretty sure I did... I'll reinstall them again and see if it works.

you should have 3 omods to install:

Better Cities Resources

Better Cities Permanent Resources

Better Cities

And please, put such questions in the official Better Cities thread. It is right here in the first page too!

As anithinks said, please use the official http://www.gamesas.com/index.php?/topic/1193172-relz-bananasplits-better-cities-thread-41/ thread to ensure a response from the Better Cities modders.

ArmeniusLOD and SceneGG have provided most of the answer however. You must ensure that you installed the entirety of the mod, not just part of it, and that you have installed all the BSA files you need, not just some of them, and that you have activated "Better Cities .esp" (which tells the game that the Better Cities BSA files exist), If you're still having problems, please use the official BC thread.