Many of the characters try to teach Oscar many things, especially Yunior, who tries to teach him how to lose weight, and many other things. How...

All the characters in the novel are like comic book characters--they have mental and physical powers that both make them great and curse them at the same time.  This is also the recipe for a tragic hero, by the way.  Both characters are on quests for a Holy Grail.  Yunior's is sex; Oscar's is love.  Each character's physical appearance hinders this quest.  Oscar cannot find lasting love because of his weight, and Yunior cannot attract a wholesome girl because of his physique (and obsession with physicality).

Whose quest is nobler?  Isn't it Oscar's?

Yunior and Oscar are foils.  Both are larger than life characters in their passions.  Yunior focuses his powers on the physical: weightlifting, physical love with the ladies, trying to get Oscar to lose weight.  Oscar's powers are internal: he has huge heart, a wondrous imagination.  Oscar wants love, but Yunior mistakenly equates this with sex.  Oscar is trapped in what Yunior calls the "friend vortex," but what Yunior doesn't understand is that one must be friends first, lovers later if any relationship is to last.  He will learn this by the end of the novel.

Yunior is limited, especially in chapter 4, by his ability to know himself spiritually, in his ability to love women emotionally, and his ability to sacrifice himself for others.  Oscar, in his titanic inner passions, teaches Yunior how to move from a superhero to a tragic hero, from one who suffers from the fuku to one who overcomes it through sacrifice, thus ending it.