Soo .... Kashiwada and Oota backstory shown that while Kashiwada is expressionless and didn't act much, she is feeling lonely and want to play with others. Oota can see that lonelyness, thats why he tried to bother Kashiwada that time.
Now, we see Kashiwada alone again, so Oota though she is feeling lonely again. Thats why he tried to bother Kashiwada again, and when he saw her happy and somewhat smiled, he thought that as a win, feeling happy, and let out a grin.
P.S. Even if Oota didn't remember, he didnt change since child, so he will probably did the same thing should a same scene occured.
And yeah, this is just a somewhat wishful interpretation.