The answer to this question is not that different than how I answer any sex question of this type. Yes, women can reach orgasm by cunnilingus. But that doesn't mean that every woman can or does have an orgasm with oral sex.
Any number of factors play into one's ability to reach orgasm. How relaxed and present you are. Your partner's ability to give you the kind of stimulation you need. Your mental state. Your health and mental health. Some medications can inhibit orgasm.
Different women have different sexual responses. Some may love the feel of their partner going down on them. Others may fear they're dirty down there or that they smell and that might not let them relax enough to enjoy the sensation of their partner's mouth on their genitals.
So, yes, it is possible to reach an orgasm via oral sex, but that doesn't mean you will, or you will every time. Want to try? Tips for better oral sex.