The most common "sins" of OBs that make their patients dislike them have got to include treating pregnant women like they are on a conveyor belt, and not treating them like real people with real questions. Some OBs do not feel that taking the time to answer their patients' questions is important. Treating women expecting babies like mere incubators, not valuing their opinions, and being condescending, are other mistakes that obstetricians make quite often. Even if they are women themselves. Perhaps the most valid reason for disliking your OB is a strong disagreement about what pregnancy and birth should be like. If you find yourself being pressurized into prenatal tests you did not want, or have sensed that your OB is not at all supportive of natural childbirth or having a doula present, and those are things you feel strongly about, it is obvious that you are not really going to get on.
So, if your pregnancy and birth philosophies are totally opposite to those of your healthcare provider, or you simply don't click, what can you do? If you are late in your pregnancy, finding another OB that will take you on can sometimes be a struggle. You can certainly try, though. You can even switch to midwifery care halfway through your pregnancy, if you can find a midwife that is not fully booked and you are low-risk. If, for some reason, you are unable to find an alternative healthcare provider, and you are stuck with the one you don't like, remember that you have the right to get answers to all of your questions (informed consent) and you can say no to any procedure you don't deem necessary, or you feel is risky (informed refusal).