A good Oc needs, I think, a good background, first of all; his story has to be pretty coherent, good, and adapted to the kind of Oc you want to do (dramatic, nothing special, joyfull....) . A good story is especially necessary if you want to make a powerful Oc or such.
Then, there is his personality: is he/she tricky? Honest? Hides who he/she really is? Willing to help people or not? Friendly, lonely, shows to be one these to while being the exact opposite? Evil? Good? Lawful, chaotic? Or the mix some of these? And if his personality helps with the events that happens to him/her, you have to show why... An Oc has to be close to real, you have to feel that he, in his/her turn, feels too.
Then the physical appearance; there, do what you want, I can't tell you xD
Finally, there is his/her abilities: what do you want your Oc to be able to do? Skilled at magic? Strong? Fast? Or just simply talented in something in particular? How will that help him in his adventures? And something very important: HOW did he get these powers/talents? By training? Was it, innate? Did he/she get them randomly? And also very important: why? Why would you give him/her special talents or powers? To be cool? Because he has a purpose nobody knows about? I don't know, something! xD
Then, there you have some elements I think are pretty important c: