oFono is the layer above the GSM firmware and has different backends for different types of GSM modems. So, oFono should be compared to FSO, not OsmocomBB, which is GSM firmware/hardware. oFono could gain a OsmocomBB backend unless of course OsmocomBB presents a standard AT interface, which oFono already supports. The other oFono backend that exists is one for Nokia modems.