A GSM OMC is basically a big collection of workstations, printers, cabling, software and database used to manage/configure/maintain/change-nappy of a GSM network. Communicating with MSC, BTS, BSS, BTC's, BSC's and much more TLA's it allows hapless engineers to see that Base Station 234.334's cables have been chewed up by nasty monkeys or large yellow dirt-moving machines.