Gemara says that you would think that without the word oso you would learn thru binyan av that seir nachshon needs tzafon like every other chatos the gemara answers that if we were learning binyan av don't talk about smicha and learn that thru binyan av so we obviously don't learn thru binyan av.
But this answer only works for R Yehuda who says that we talk about smicha by seir nachshon not according to R Shimon who says we don't talk about Simcha by seir nachshon - so why can't we say we need the oso so as not to learn the binyan av according to R Shimon?
Yossi b, Monsey ny usa
(a) The Sefas Emes (Zevachim 48b) actually suggests exactly what you did - that according to Rebbi Shimon 'Oso' is indeed necessary, since Rebbi Shimon learns Sha'ah me'Doros (and he indeed requires Semichah by the Se'ir Nachshon based solely on a Binyan Av).
According to this interpretation, Mar Zutra is rebutting Ravina's suggestion that 'Oso' is necessary according to Rebbi Yehudah. To the contrary, he contends that only according to Rebbi Shimon is 'Oso' necessary, because it teaches that we do not require Tzafon by the Se'ir Nachshon based on a Binyan Av.
(b) However, the simple reading of the Beraisa is that Rebbi Shimon argues with Rebbi Yehudah and does not require Semichah by Se'ir Nachshon (and hence, he does not learn Se'ir Nachshon from a Binyan Av). Had he learned Semichah of a Se'ir Nachshon from a Binyan Av and argued on the fundamental principle of Rebbi Yehudah that we do not learn Doros from Sha'ah - he would have made a remark to that affect (such as "l'Se'ir Nachshon Eino Tzarich, u'Mah Talmud Lomar..." - see for example Berachos 48b, Shabbos 133a, Pesachim 67a).
Mordecai Kornfeld
Kollel Iyun Hadaf