An oversight is a mistake you make when you're not paying full attention. Your failure to add the sugar to the cookies was an unfortunate oversight — you were so engrossed in texting that you forgot it.
Oversights are not intentional mistakes. Usually they're just the result of inattention. If you make an oversight in your inspection of a nuclear power plant and it later goes into meltdown, you'll get fired. An inspector is supposed to pay attention. But if you forgot your cousin's birthday because you're busy taking care of Grandma, that's a forgivable oversight. Oversight is also the act of supervising something, like the construction of a dam, or a high school prom.