Are omeprazole and other members estrogenic and/or antiandrenergic?
There is not a lot of evidence that the compound itself is estrogenic, but it may affect how estrogen is metabolized in the body, leading to feminizing outcomes.

Pubmed is a good source for information in the biomedical literature- here is a pubmed search that has more information on those terms.

Looking through some of the results of that search, I find that there seems to be a fair amount of evidence that omeprazole has something to do with gynecomastia, but the association is not strong:


A review of the literature has some conflicting information. One study of general practice in Britain suggested no detectable statistical relationship between taking omeprazole and gynecomastia. However, a smaller study suggested that in people who experienced gynecomastia who were also taking omeprazole, removing omeprazole helped in the majority of cases. I suggest reading the section of that review on omeprazole. The conclusion of the review is that omeprazole probably has a role in gynecomastia.
There is a literature of studies (mostly in Spain?) ascribing omeprazole a role in gynecomastia. One example.
There is evidence that omeprazole has a chemical effect on estrogen metabolism. However, it was much less strong than some other drugs examined. 
There is evidence that omeprazole slows the metabolism of estradiol, an active estrogen compound (see Figure 2 in the review linked above).


Overall, people seem to say it (and related compounds) probably has something to do with gynecomastia, but it is not a principal contributor on average. Possibly it shows a synergy with some other drug or some lifestyle variable.