Surprisingly there's people who are kind of lost with all these modern internet abbreviations and such like.
That said OMCD means OMG when said by these stupid people. Although Oh My God doesn't actually abbreviate to OMCD in any way, OMCD and OMG have shared letters and thus they are intermixable.

OMCD also associates to several company names.

Oh My Charles Darwin, a form of OMG for evolutionists.