Slovenia international Oblak signed fresh terms on a four-year deal with Atletico last week, the new agreement reportedly including a release clause of around €100million.
The former Benfica goalkeeper, 26, has spent the past five years in the Spanish capital having succeeded Thibaut Courtois as the club's number one in 2014.
Simeone compared Oblak's new deal to Antoine Griezmann's contract extension last year in terms of its importance to the Rojiblancos moving forward.
"It is very important for the club and a great decision - as it was with the Griezmann one last year - on staying with us," Simeone told reporters at a news conference to preview Valencia's trip to the Wanda Metropolitano.
"He is probably the best keeper in the world.
"It is a big joy for the club and for the fans because Jan is an extraordinary goalkeeper.
"It is something which is very good news for the club, that's for sure."
Barcelona will be confirmed as LaLiga champions should they beat Deportivo Alaves on Tuesday and Atleti then lose to Valencia the following day.
Copa del Rey finalists and Europa League semi-finalists Valencia have found form in recent months, losing just one of their previous 22 games in all competitions, as they chase a top-four finish.
They won only one of their opening 13 matches but Simeone always expected Valencia to turn their season around.
"I think this last period of the season is what they planned at the beginning, according to the squad they have," he added.
"I am not surprised seeing how they are improving because they are a solid and organised team with a clear concept of the game and good replacement players.
"So, being at this position in the table, playing the semi-final of the Europa League and [having] qualified for the final of the Copa del Rey describes how good their squad is."
Simeone, who remains without the suspended Diego Costa, will have defender Stefan Savic available again after he returned to training on Tuesday following a hamstring injury.