Phil's Diner is a diner that Oskar worked at for a time under the alias of Seth.
At some point after returning from Ecuador, Oskar started working at the diner under the alias of Seth. Using the skills he picked up in Ecuador, Oskar roasted the beans for the diner's coffee himself.
After learning of Oskar from Olivette, Crowley managed to track him to Phil's Diner. Posing as a customer, Crowley visited the diner to meet Oskar and the two chatted as Oskar served Crowley. Their conversation was interrupted when Crowley received an apparently urgent message from Dean Winchester and had to leave.
After learning that they needed Oskar for the Mark of Cain Removal Spell, Crowley returned to the diner and put all of the patrons and staff to sleep. Telekinetically pinning Oskar to the counter, Crowley relayed what he had learned of Oskar's history with Rowena MacLeod from Olivette to the young man before kidnapping him.