Onoskelis is a character in Afterparty that is very useful to meet, but it's entierly possible to actually skip meeting her entierly, so you are in the correct place. You can meet her if you haven't already been to Satan's Party, but if you've already been inside the party, you're out of luck.
When you are at the door to Satan's Party, you will be given two different choices by the secity guard as to how you can enter the party. You must select the option where you help a human called Lynda get a Margarita if you want to meet Onoskelis. This quest will lead to you an island called Little Rantalia, and when you are here, look for a bar called Durdy Hurdy Gurdy. Order a Margarita here and give it to Lynda, which is found in the right side of the club on a couch.
Lynda will give you a new quest, and you will need to do this. This quest will allow you to go down into the basement of the bar, and on the very right of the basement you will see Onoskelis.
This is the only time you will be able to meet Onoskelis (Ono) in the entire game!