What is the meaning of the word "O"?
Rashi: It means 'If'. 1
Seforno: It means 'or' 2
What are the implications of "O Hoda Elav Chataso? What did he discover?
Rashi: When he sinned, he thought that what he was doing was permitted, and he discovered that it was in fact, forbidden.
Ramban #2: Refer to 4:22:3:1.
Refer to 4:22:2:3. Da'as Zekenim, Moshav Zekenim - this is written also about Chatas Hedyot, but not about Chatas Kohen Mashi'ach of Chatas ha'Kahal, for they do not bring Asham Taluy.
Except that the Ibn Ezra learns that he himself is aware that he sinned by inference (See Ramban).
Why does a Nasi bring a Sa'ir?
Moshav Zekenim, citing R. Eliezer of Garmaiza: The Shevatim (Yosef's brothers) sinned through a goat (they put its blood on Yosef's cloak - Bereishis 37:31). The Gematriya of Nasi (361) equals that of Shevatim, therefore he atones with a goat.
"Korbano" is extra. What do we learn from it?
Moshav Zekenim citing the Rosh: He should bring a big, fat Korban, since every day he eats fattened animals. His table should not be full, and Hashem's is empty (i.e. is less honored than his).