The author has proven that he is somewhat following the seven warring states history. Of course, this is a manga so characters and situations have been altered or exaggerated, but if we follow history, we can somewhat predict what's going to happen in the future. The historical figure that Ou Sen is based on actually goes down as one of the greatest general of Qin and helped conquer the rest of the states. By this, I can infer Ou Sen would be instrumental for Qin and their ambition, since that's the plot of the manga, so him betraying Qin doesn't seem likely. Unless the author has a serious change of facts for shin whose historical figure he is based on goes down as the greatest campaign failure in the conquering of other states. By this I mean the author might swap places of Shin and Ou Sen fates, where Shin becomes the greatest glory for Qin but I really don't see that happening.