In the Kill Bill movies, where did O-Ren learn to be an assassin, and when did she actually join Bill's squad?
This is not an official source, but it matches what I recall from seeing the original movie. The Kill Bill Fandom Wikia has the following to say about the specific topics you mention:
It is presumed that her very early childhood was untroubled as little is known about her infancy. Her father was a Sergeant Major in the United States Marine Corps and her mother was a faithful housewife.
However, at the innocent age of nine, her life had forever changed when her home was intruded by assassins led by the ruthless crime boss, Matsumoto, Japan's cruelest Yakuza boss.
Both her parents were killed by Yakuza mobsters and their boss while she hid under the bed. They later set their house on fire and Ishii barely escaped
After spending 2 years training in assassination and marksmanship, O-Ren, now aged 11...
That part is in the movie and doesn't bear repeating further details that don't answer your question.
Her training was at a young age in Tokyo and hence, unlike The Bride or Elle Driver, did not train with Hanzo or earn a sword from him.
Five years later and aged twenty-five, O-Ren was a member of the Deadly Viper Assassination Squad and took part in the vicious attack against the bride at her wedding rehearsal
So there is a fourteen year period between her training and the start of the film.
9 years later, at the age of 20, O-Ren had become one of the top female assassins in the world and was seen assassinating a Latin American political figure with just a single shot from her position of a rooftop.
It is not clear if this assassination is part of Bill's Deadly Vipers missions, but I will assume it is not.