Cracks at corner of mouth after oral?

Friction, drying. I assume you mean after oral sex you find cracks in the corners of your mouth. It's not serious, in all likelihood. There is a small chance that you have a sexually transmitted disease on your lips, and that requires treatment. But more likely is that the friction and motion of the sex act, plus perhaps the drying quality of genital secretions on your lips, combine to cause the cracking. Lubricate.