In Season 1, Eric van der Woodsen is doing time at the Ostroff Center because of an attempted suicide. He is kept there by his mother until she realizes how much he hates it there, enough to agree to be jailbroken by Jenny and Blair so she takes him home.
It isn't until Season 4 that another character goes into the Ostroff Center. This time it is Eric's older sister Serena, she goes there after Juliet drugged her and made Serena think she had used it herself. Dan takes her back to the loft but she goes back to the Ostroff for a short time until she leaves again to sort everything out with Juliet.