The Ostroff Center is a private psychological instution and rehab clinic on the Upper East Side. Known for it's discretion, it's considered the best rehab facility on the UES.
In the episode Poison Ivy, Chuck and Blair spy on Serena, wanting to expose the real reason she came back. Chuck manages to take photos of Serena leaving the clinic after a family session with Lily and Eric and tells Blair. She decides to make a donation to the center on behalf of Constance Billard as a way to thank them for the wonderful job they do with helping drugs addicts, namely Serena. She plays along to protect Eric, but Lily is deeply ashamed and worried for Serena's future. After the speech, Eric confronts Blair and tells her that Serena isn't the patient, he is. He shows her the scars from slitting his wrists, and as a result, Blair and Serena make up. At the end of the episode, Jenny visits Eric in his room, where she sees flowers sent by the Brown University rep because his cousin also did time at the center.
In Dare Devil, the Ostroff Center appears again when Blair dares Jenny at the annual Waldorf sleepover soirée to help Eric escape for one night after Lily decides not to let him come home for the weekend. Jenny helps Eric while Blair keeps the staff busy by playing a stoned drug addict. Eric gets home safely to the clinic after a night out. Meanwhile, Lily is desperate to find Eric once she hears he escaped. Dan and Serena are on their first date but pause things to find Eric. When they do, Eric confesses that he's lonely with only Serena and Lily to talk to, and he's really ready to come home. After he returns to the center, Lily comes by and brings him home for good.
The clinic appears again in Gaslit, after Lily involuntarily commits Serena after she was brought to the hospital after a drug overdose unknowingly caused by Juliet. While there, Eric says her room was bigger than his. Later, she has Dan break her out but they're caught by Blair and Lily before they can leave town. A Gossip Girl blast contains a photo of "Serena" (Juliet dressed as Serena) doing cocaine and Serena realizes that maybe she really did go on a binge. She lets Lily take her back to the center, and the family all has Thanksgiving dinner together. In The Townie, Serena goes though therapy sessions at the The Ostroff Center, starting with not talking or let any friends or family visit for the first days she spends there. When talking to the shrink, flashbacks from her time at the boarding school are shown. Later, Juliet breaks into Serena's room. Serena desperately asks about her motive, leading to her discovery that her mother forged her signature on an affidavit claiming that Ben Donovan, a boarding school teacher, had statutorily raped her in order to get her back into Constance.