I think that people are missing something here. Tsukishima scored a point and Kageyama complimented him, Hinata scored eyes opened a point even though they thought that he would act as a decoy, Tanaka scored a point...
Oikawa is very talented but he is arrogant and I think the difference lies within what's happening around the setter. Kageyama is at this moment the setter who has the reasons to become angry because nearly nothing works for him at this moment. Kageyama is keeping his cool because his teammates assured him that everything will turn out good.
Let's wait and see because I believe that Tanaka, Tsukihima, Hinata, Nishi, Asahi, Daichi,... are going to strengthen Kageyama's setter role. But when Oikawa('s team) is going to fail then he'll become angry and he'll fail. Oikawa is doomed to fall of his pedestal. He'll face Karasuno's true strength 'teamwork' and at that moment he will start to act like kageyama before he joined Karasuno. Who will give efforts to pick up oikawa's ball then