O when mine eyes did see Olivia first..... That instant i was turned into a hart.
He is so deeply in love with Olivia, that he says he is a "hart". This implies that he is a wild animal, and that he is "free".
point. orsino is deeply in love with olivia.
And my desires like fell and cruel hounds
His love for Olivia, and his desires are chasing him and driving him crazy, like hunting dogs. The use of the word "hounds" relates to the word "hart" in the previous line, because a hart is a male deer.