You have just realised that you do not need a skill anymore? From now on you may utterly forget it! You are able to forget only those skills, which are lower level than your oblivion skill. The oblivion – unlike any other skills – does not increases the development cost of skills. You can use the skill oblivion once in a week.
When forgetting a skill, you receive approximately half the SE you spent on it. You msut forget 2 skills of at least level 10 for a quest later in the game, so it's best to think about what you'll get rid of early on. For that quest, it is recommended that you train oblivion to 5 then mind boost it when needed (to save SE).
Learning Level: 32
Max level: no limit