#1
just read through Oleo. I know that this song is based on the rhythm changes, and that as a jazz major, its a very important progression to know.

id just like an explanation of its importance. i havent fully analyzed it, but from the read-through I just did, i can see that there is some ii-V happening, so im not totally lost.
#2
Oleo's chord progression travels pretty exclusively in fifths. The "rhythm changes" are so called because the song is based off of George Gershwin's song "I Got rhythm."

It was common for bebop performers to take older chord progressions and add another melody and more chords to give more soloing options. I have a feeling that rhythm changes is so popular because it moves in fifths through the whole song (changing key from Bb major to Eb major to C minor and then back to Bb major again), making it easy to improvise with little rehearsal time.

You're right that there's ii-V-I, it's pretty much how the whole song moves, going I vi ii V I. The B part starts on the III chord of Bb and it cycles in fifths before going back to the I.

Hope this helped.