So it is finally revealed why Miki hates Oga with a passion. It all dates back to their middle schools years. Miki was getting bullied by some guys from a rival school and Oga intervened. All the guys got beat up by Oga and then came back for revenge. Miki wanted to join in the fight, but it would seem that Oga had already decided that it would not be a good idea for Miki to fight. Oga decided to take a brute force method and knock Miki so that way he could not fight.
I can understand why Oga did what he did to Miki. When Oga first said to Miki that he did not know him Miki should have taken that as Oga trying to protect. Oga did not want Miki involved with the fight because Miki was a rather weak person at the time. Oga did not want to have to spend his time trying to protect Miki’s ass while trying to fight. But when Miki followed Oga realized that unless he did something to incapacitate Miki, Miki would have ended up with a lot more damage than what Oga inflicted. It kept Miki out of the fight.