TV Series and the actors appearing in them are nominated for categories in the Golden Globes. So why isn't there a category for TV Series in Oscars? Is there a special policy regarding this?
Because Oscars are awards given by The Academy of Motion Picture Arts and Sciences (AMPAS). A Motion Picture in this case refers to Movies/Films and they are (per wiki)
to recognize excellence in cinematic achievements in the United States film industry...
Golden Globes, by contrast, are (also per wiki)
accolades bestowed by the 93 members of the Hollywood Foreign Press Association, recognizing excellence in film and television, both domestic and foreign.