Just to clarify: Omega E enlargers are 5x7 enlargers; D enlargers are 4x5 enlargers. Chromega is the color head; again, D is for the 4x5 head, E, for the 5x7 head.
Do make sure which enlarger chassis you have and which color head (a D head on an E enlarger might be problematic, but maybe okay if you're only enlarging smaller negatives). Also, be aware that there are film-format-specific light mixing boxes for the color heads. You can enlarge smaller negatives with a mixing box designed for larger ones (with less efficiency do to light loss), but not vice-versa, since there will be no coverage.
Basic optics about lens-to-negative and lens-to-baseboard distances apply here. If you can't focus your image on the baseboard, you need to change the lens-to-negative distance to compensate (cone or recessed board or whatever depending on the nature of the problem).
The Omega enlarger system, especially for the older enlargers, included lots of things to space the lens correctly in relation to the negative depending on format. A bit of research will help you understand the ins and outs.
Have fun,
Doremus