There are quite a few prominent management companies in Carlsbad that would cover Oceanside. Our company, Jacques Real Estate, is in downtown Oceanside and we also cover Carlsbad. Most our clientele is in Oceanside however. Helen Vogel would be your contact person. Phone number 760 757 0700
I would venture to say that Ranch and Sea are the most prevalent and well known property management in Oceanside. I would definitely suggest shopping around and weighing the pros and cons of local ones in the area. Mike gave you a great starting part with those website links.
I don't know them personally, but if I were looking I would probably start calling and comparing these ones first. These ones apprear to have the strongest presence, so It would be worth it to me to investigate.