Why does my girlfriend only cum during oral not intercourse?

Most women need clitoral stimulation to reach climax. During oral sex it is easier to maintain clitoral contact. You can help her reach climax during intercourse a couple of ways.

One would be to have her on top so she can stimulate her clitoris  against your pelvic area. You can also use a vibrator or finger to massage her clitoris during intercourse.

It's also important not to pressure her to reach orgasm during intercourse because this will make it near impossible. Take you time, experiment with adding any kind of clitoral stimulation to enhance intercourse, and if it does not work out that she is able to reach orgasm during intercourse, know you can use intercourse as foreplay for her orgasm and finish your encounter with oral sex.