Alanis Morissette/WMG

"You Oughta Know" is about...

 * Dave Coulier, who played Joey Gladstone on Full House. He started dating Alanis in 1992, when the two met at a hockey game. Coulier himself believes that the song is about him, claiming that the lyrics hit very close to home.
 * I second Coulier. I mean come on, it's Uncle Joey here, and heck, he says it's about him. Case closed.
 * Alanis has specifically disavowed this one, however, word is that her song "Hands Clean" is about her relationship with Coulier.
 * Leslie Howe, the producer of Alanis' first two albums.
 * Bob Saget, also of Full House. He denies having ever been in a relationship with Alanis, and agrees with the idea that "You Oughta Know" is about Coulier.
 * Matt LeBlanc, who played Joey Tribbiani on Friends, and appeared in one of Alanis' early videos.
 * All of her past exes. It's not about one man in particular, but about all men.
 * Me.
 * Tupac Shakur. It turns out that they had a secret fling (sometime in 1994 or '95, before the "Jagged Little Pill" album came out) that Alanis thought was something more than a one-night stand. When Tupac rejected her further advances, she became enraged and wrote this song. Worse, over a year after "You Oughta Know" came out, Alanis was still unhinged and insanely angry at Tupac...so she decided to punish him. That's right, Alanis Morissette is either the one who shot Tupac, or she at least hired the person who did. Writing an angry song wasn't enough. She wanted brutal revenge.

Robin Scherbatsky is an Alternate Universe version of Alanis.
Both were Canadian teen idols in the early '90s. They both had over-the-top '80s fashion when they were famous (The Eighties reached Canada late). Both have disowned their teen pop pasts and become a lot darker and snarkier since. Robin was in a relationship with a man who appeared in one of her music videos, and the man that inspired Alanis to write Jagged Little Pill was (according to rumor; see above) someone who appeared in one of her videos. Robin is basically Alanis if she didn't continue making music.
 * This is verified by the commentary on How I Met Your Mother, when the writers say in the Robin Sparkles episode they were thinking about bringing Alanis on and having them be a pop duo together in Canada, but it never worked out.

Isn't It Ironic? IS actually ironic.
The lyrics are decidedly not ironic, even though the song suggests that they are. This was her goal all along: ironically, the song that is so often called out for not being ironic when it is supposed to be ironic is ironic simply because the song is a failed attempt at irony. After all, Isn't It Ironic? that "rain on your wedding day" isn't ironic even though she claims it is? Well played, Alanis Morissette.