(a) Your question is asked by the Beis Lechem Yehudah on Bava Kama (by R. Yehuda Meir Dvir Shlita).

(b) The Sukas David (by R. Dovid Kviat Shlita of Mir Yeshiva, NY) Bava Kama 5a #10, answers that indeed there is nothing forcing Rav Oshiyah to agree with Rebbi Akiva on every single dispute. However, when the Gemara asked that Rav Oshiyah should hold with Rebbi Akiva on a Tam that injured a Man also, the Gemara did so because it knew that it could answer that Rebbi Akiva had in fact retracted from this, and therefore it emerges that Rav Oshiyah sides with Rebbi Akiva on everything, which is a better answer because it means we have fewer disputes. (Don't forget that "Talmidei Chachamim Marbim Shalom ba'Olam" - i.e. we always try to minimize the number of Halachic disputes that exist, wherever possible).

The Sukas David adds that a similar explanation can be given according to Tosfos below 7a DH v'Rebbi. The Gemara states there that there is a different Din for an ox belonging to Hekdesh than an ox belonging to a private individual. Tosfos asks, "If so, our Gemara on 5a could have asked that Rav Oshiyah should have listed 2 different kinds of oxen." The Sukas David writes that, again, Rav Oshiyah is not forced to agree with Rebbi Akiva on every dispute, but rather Tosfos merely meant that the Gemara should have at least asked this question, even though it could still have answered that Rav Oshiyah & Rebbi Akiva differed.

(c) I would like to add some further ideas here, which I think will merely explain a bit more the answer of the Sukas David. This is based on the Gemara Kidushin 63b which states that it is better to make a forced explanation in a Mishnah, but keep the Mishnah going according to only one Tana, rather than explain that the Mishnah is discussing only one case, but goes according to 2 Tana'im. We learn from this Gemara that wherever possible, we try not to make different opinions in the same Mishnah.

Now, it is true that the Beraisa of Rav Oshiyah that the Gemara cites here 4b, is not actually a Mishnah. However I argue that its authority is not much less than that of a Mishnah. My argument is based on Chulin end 141a, that any Beraisa which was not taught by Rebbi Chiya and Rav Oshiyah is not reliable and one cannot ask a contradiction from such a Beraisa. Rashi Chulin 141b DH Rebbi writes that Rebbi Chiya and Rav Oshiyah arranged the Beraisos and were careful to cite exactly what each Tana said.

This suggests that even though the authority of Rav Oshiyah's Beraisa is not quite that of a Mishnah, nevertheless it is not far off. Therefore it is possible that just as one assumes that it is unlikely that a Mishnah represents more than one opinion - because the Mishnayos are very carefully compiled and are not usually a composition of different opinins which have been stuck together - so also a Beraisa of Rav Oshiyah can probably be assumed to represent only one opinion. This is why the Gemara here asked that if Rav Oshiyah held like Rebbi Akiva concerning Edim Zomemim, it is likely (although not 100% certain) that he held like him concerning Tam that injured Man. Even though there is no logical connection between the 2 issues, nevertheless it is probable that Rav Oshiyah (who was on the borderline between the period of the Tana'im and Amora'im) was consistent with his opinions. Rebbi Akiva lived a long time before Rav Oshiyah and it is likely that Rav Oshiyah did not jump around so much, taking one opinion from this Tana and another from a different Tana. Therefore the Gemara asked that it should be most logical (even though not totally essential) if Rav Oshiyah agreed with Rebbi Akiva on each dispute.

Behatzlachah Rabah

Dovid Bloom