Both Omnicef and Cephalexin are drugs in a class known as cephalosporins. They are NOT considered to be of the penicillin class although there is a SMALL chance of cross reactivity. Current data suggest that the rate of cross-reactivity between penicillins and cephalosporins is probably less than 1%.
In general, the rate of allergic cross-reactivity is highest between penicillins and first-generation cephalosporins (cephalexin is a first generation) . The chance is MUCH less with later generation drugs (Omnicef is a later generation).Seeing as how you were OK on cephalexin, I must imagine you should be just fine on Omnicef.