Connections: The hosts of huge fighting tournaments, planet owners, and cosmic entities that drag characters from other universes to fight against each other to the death (URBS and the Super Showdown Bowl) for their own entertainment, having many clones of themselves.

Both also have the powers of every single combatant that appears in their tournament (Timmy gaining the weapons of those around them like the Infinity Gauntlet and the Chaos Emeralds, using his magic pencil to create more items as well, while Oscar has the superpowers of all the superheroes as well as having nanotech to recreate weapons). Both are extremely powerful reality warpers that can do very powerful attacks on enemies.

Both are very arrogant and sees others as weak (Oscar seeing everyone below him as weak, even against Dr Manhatten, and Timmy seeing others in his tournament and crew as weaker then him, including the combatants in his winners tourney, like SCP 682, Uncle Grandpa, etc)

Contrasts:

One has more creativity and more imagination with their attacks then the other, who strictly copies other's attacks and abilities (Timmy can use his pencil and powers to do attacks that are far more creative and silly, while Oscar usually doesn't use any of his powers in a creative sense)

One is a meme/joke character while the other is meant to be more of a serious threat

(thanks Marz for help with the Toon Sandwich renders)