Went to walthers.com, did a search for O scale, structure and added search word "background".
Found this one: http://www.walthers.com/exec/productinfo/933-2717
Don't know if that was what you were looking for ?
Smile, Stein
Try this too:
http://www.choochenterprises.com/html/ultra_scale_structures.html
http://www.dpmkits.com/
DPM also makes an O scale modular system you can use just to build back ground buildings.