Okay, last episode was great but I’m disappointed that they did not animated this panel :
It’s just before Tobio’s last toss to Hinata. Oikawa is here, he know what’s going to happen, he knows that Karasuno is going to win… And he smiles.
It was already obvious that he doesn’t want Shiratorizawa and Karasuno (id est Ushiwaka and Kageyama) to “both lose”. In chapter 148 he’s clear about it :
He calls Kageyama “my kouhai”, as if there still was a link between them ; he can say to Iwaizumi “he’s not my student !”, he still gets a little smile after.
It’s like a succession. Oikawa didn’t succeed in beating Ushiwaka, so Tobio has to do it for him.
For Oikawa, Kageyama becomes a way to take revenge on Ushiwaka. After all, they are the same type of setter, unlike Shirabu’s play, and Kageyama uses the same moves as Oikawa throughout the game : the “I’m going to spike but in fact I toss” and “I aiming at the setter, now he can”t toss”.
Their resemblance and connection is even mentionned by Shirabu “the sort of things you’d expect from Oikawa’s kouhai”. Tobio is more seen as “Oikawa II” than Kageyama. It’s also Tobio’s goal, to be recognized by Ushiwaka as a setter like/better as Oikawa.
And Oikawa wanted to come alone to the game because it’s something personal.
This last smile reveals how involved he is in this game, even if he can’t play. He was rooting for Kageyama even before the beginning of the game. He knows that Tobio’s last toss will make Karasuno win, and for him it’s a personal victory as well.
And of course you can read it as a mark of pride towards his kouhai, I know many of you needed a proof of it.