I'm a little confused - in your country of origin, do urologists rountinely manage gynecologic malignancies? Is this not the realm of, well, gynecologists?
If you are truly concerned about liability issues in terms of accreditation, I think the bigger issue would be whether you would be able to actually develop a surgical oncologic practice. From my experience (granted, limited to large academic centers in major cities), gyn onc is the exclusive realm of those who have completed a formal fellowship. Gyn onc is not an enormous field, and given the low demand for gyn onc, I doubt you would be able to build a practice with board certification in general OB/Gyn only. Perhaps this is different for rural areas, and if this is so perhaps someone with more knowledge of this could pipe in.
Also, standards of management may be different in the U.S. than in your country, and in terms of malpractice, you *would* be held to the standards of practice of the U.S. Exceptionally risky to jump into a surgical practice with the sickest possible patients, based only upon experience built in another country!!! (baaaaad idea)
Also, as I've mentioned already, gynecologists routinely perform surgical treatments of incontinence - if your background is in urology, you can most certainly build a general gynecologic practice with an emphasis in uro-gyn types of surgery, without doing a fellowship. Uro-gyn is not a boarded fellowship anyway, so this is done all the time. Compared with oncology, incontinence and POP are much more common, so the potential patient base is greater, and you would not need to complete a fellowship.
And hey - you don't have to get into the pissing matches RE: "are gynecologists real surgeons" crap. Gynecologists get plenty of surgical training for the procedures that we perform. Your experience will also depend on your attitude and the purposefulness with which you select a residency program - surgery was highly important to me, so I ranked programs with more surgical training time highest.
Also, some advice - if you maintain and publicize the attitude that "I was forced to go into OB/Gyn because it was easier to get into than the field which I would actually prefer to be in" I don't think it will go over very well. People who choose OB/Gyn do so because they love the field - because of its surgical as well as primary care opportunities, interest in reproductive health/women's health, etc. And when programs pick applicants for residencies, I imagine that they will select those folks who demonstrate genuine commitment to the field. Your background in urology will help you, but if you're actually going to go into gynecology, you're going to have to at least pretend to be interested in *women's* urologic issues or people will have some questions