As others have said ... you can take any certification from any agency without cross-over requirements up until you reach the professional (divemaster) rank.
A bit of advice ... at your level, focus more on finding a diligent instructor, rather than what agency he or she teaches for. AOW curriculum can vary wildly, depending on the instructor teaching it ... those who teach to the minimum standards (in any agency) won't really teach you a lot, so much as simply take you on a few guided dives. Those who are interested in really teaching will focus on new skills and knowledge ... then take you diving.
Interview the instructor before signing up for the class ... find out what the class involves. If it's just diving, with little or no class work, don't waste your time ... regardless of the agency.
... Bob (Grateful Diver)
NAUI Instructor 41751