As best as I can tell, the following is true. "The only organizations as an undergraduate student that you could belong to that would affect your ability according to by-laws to join an NPHC sorority (either later as a undergraduate or as an alumni) are those which are allowed by law to be women only". Since Omega Phi Alpha as well as other non-social sororities such as Tau Beta Sigma and Delta Omicron allow men, they should be OK.
The oddball exception here appears to be Sigma Alpha Iota Music Sorority. It is a member of the Professional Fraternity Association (but not NPC or NPHC), but has a Title IX exemption to only admit women (Like Phi Mu Alpha's to only admit men). According to
http://www.greekchat.com/gcforums/sh...56&postcount=9 , one of the posters has a chapter sister in both SAI and DST.
What individual sisters would think in terms of membership in specific organization, you'd have to ask them...