Endrick struck for the 1-0 Copa del Rey semifinal first-leg win in San Sebastian on the night.
Real pair Raul Asensio and Vinicius Jr were barracked by a section of the home support.
"As in many stadiums, what a few people sing does not tarnish the rest of the fans. Insults are condemned, of course. We don't like them. It is not the right way and we join the cause that it is not the way," said Oyarzabal afterwards.
On the game, the midfielder added: "We lacked accuracy. We had enough chances, so did I. They don't forgive you for the slightest chance they get. We didn't play a game to lose, we deserved at least a draw.
"Football is like that. It's a shame, but there's still the return leg to go and we're confident we'll do just as well as we did here."