First off, Oscar is gay. He came out in season 3, after Michael outted him by calling him faggie (he meant that he was being stupid).
Oscar was going to quit after 
Michael forced him to (awkwardly) kiss in the office. However, Jan offered him 3 months off paid, and a company car, in
return for his refusal to sue the company.
Oscar returned to find that Angela, with her Conservative Christian beliefs, was now kind to him and wanted him to join the Party Planning Committee. In the meantime, Oscar still puts up with Michael's ignorance ("your gayness is not what defines you... it is your Mexican-ness"). Yes... you read that right.
Oscar, Kevin, and Angela were part of a mini-series called "The Office - The Accountants" that aired during the 2008 Writers Strike in Hollywood. $3000.00
had gone missing from the books, and 
they set about trying to find it. The obvious suspect was Michael, who probably logged it as a business expense
at Hooters, but it eventually turned out to be Angela's fault, as she logged an equipment depreciation twice! Said Kevin, "This is the best day of my life!"