basically whatever sexual thing you do to your partner's genitals is considered oral sex.
for girls: when a guy(or girl) tongue fucks you, licks your clit, and eats you out.
for guys: when a girl(or guy) gives you a blowjob, or plays with your balls using their tongue.
I came back from college class that day pretty tired and dejected. I had gotten a D on a test, and my dad would kill me. When I came in my apartment my boyfriend Troy was inside. "Hey," he said. Troy had medium brown hair, tanned skin, and grey eyes. His features were perfect, with a sturdy nose and a hot smile. "What's wrong? You look kinda bummed out."
"Yeah," I replied,"I got a bad grade. Dad'll have my hide."
"Oh," he said. "I bet I know how to make you feel better."
"How?" I asked.
Then Troy got up and put his strong arms round me. He told me to get on the bed. I blushed and had some doubts but complied.
"I know you don't want to get pregnant," he said. "So we're just gonna have some oral sex."
I agreed. Slowly he ran his hands through my hair while dry humping me. Then he took off my shirt and bra. He squeezed my boobs. Then, slowly and seductively, he took off my pants and undies. He kneeled and licked my stomach. Slowly he went down. He licked my inner thighs and went on to my pussy. "Mmmmm.." Troy said while taking his tongue in and out of me. One hand went up to my boobs and the other tickled my clit. It was slow first, but then he went harder. "Mmmmmm!!!!" Troy yelled. "AAAh GOD!!!!!" I screamed. I felt myself explode. "tasty!" Troy remarked as he licked my sweet warm cum of of me. Then I violently grabbed him and shoved him on the bed. His huge throbbing cock was already erect. I licked and sucked and sucked. I was playing with his balls while swirling little circles on the tip of his penis using my tongue. I tortured him for sooo long and he couldn't handle it. I kissed his cock and cupped my hands softly around it, hugging it. Troy was swaying. Then I looked and him seductively and smiled. I then wrapped my lips around his cock and pulled his shaft. Slowly I went back and forth on his cock like a lollipop. Then I went faster, faster, faster. I could see Troy was struggling to hold it in. I looked up at him and smiled while sucking and fluttered my eyelashes. That was it. His mouth went agape and suddenly my mouth flooded with his cum. It was hot and sticky. He then took some cum and put it all over my face and boobs. I put some on his chest. I licked his cum right off Troy's penis. He sighed. I placed my pussy on his belly and rubbed and rubbed and rubbed. He licked his own cum off my face, including my mouth, and my boobs. Troy liked to pay special attention to the nipples. I did the same to his chest. Then we went to sleep after a great oral sex session.