Casillas opened himself up to a Twitter question and answer session on Sunday, and was asked who was better out of the Atletico and Barca stars.
"Honestly, analyzing the current moments of the goalkeepers on a global level, I think they're the two best," he wrote.
"I mean, they're having a sweet moment."
Oblak, the 25-year-old Slovenia international, has conceded just five goals this season and has also let in just 109 goals in 174 appearances for Atleti.
Ter Stegen grabbed the headlines after a sensational showing against Sevilla, where he made two double saves, and has conceded 143 goals in 153 games for Barca.
Casillas spent a total of 25 years at Santiago Bernabeu and is one of the most decorated goalkeepers in world football.
He has since won the Portuguese Premier League with Porto, and was Spain's goalkeeper during one of the most glorious eras in the country's footballing history, winning two European Championships and the World Cup.
And he has yet to close the door on a return to the national team, though Manchester United's David dte Gea is currently La Roja's No.1.
Asked if he would like to return to Spain action, he added: "Of course! I understand that Luis Enrique has his new ideas for this new stage but I have my hope to return!"
Casillas has also urged Real's fans to remain patient after their worst start in 17 years, with Julen Lopetegui's men without a win in five games.
"Stay calm," he said. "The season has just begun. The fans are demanding but also sympathetic. There have been important changes."