The pair were team-mates for the past two seasons at Celta Vigo, where Oubina was very much the local boy made good.
Keen to strike out in the Premier League after Vigo's relegation, Oubina spoke to de Ridder about Blues before his loan was ratified.
"He is a very stable player," said de Ridder. "He doesn't lose the ball often.
"Physically he is strong enough and he has good vision.
"So he is a controlling player, but he can also see through passes.
"Overall, he is a good midfielder and I do think he will fit into the team and the league here."
De Ridder added that Oubina would be no trouble for boss Steve Bruce.
"He doesn't speak English, but he's very quiet and a very nice person. He's very balanced out," he said.