Office holiday parties are often used to see and be seen and politics can play a big part of the affair. If it is a big company, people tend to hang out with members of their own department. Accounting folks don’t mix with the advertising department, etc.
You, as the planner, need to find some office Christmas party games that will get people working together for a common goal.
Charades is always popular. Make teams comprised of people from different departments. One team member draws a slip of paper that has the name of a Christmas movie, song, poem, or TV special. She has to act out the title or phrase while her teammates try to guess. The first team to guess correctly wins the game.
Another fun game is musical chairs. Use a Christmas song. This game can be hysterical when folks have already sampled the Christmas cheer just be sure they haven’t overdone it and nobody gets hurt.
Even gift giving can be fodder for office Christmas party games. Play "It’s a Dirty Shame". Folks sit in a circle around a pile of wrapped gifts. Pull a name from a hat(boy, etc.). That person says her name chooses and unwraps the gift. Play moves to the right. Then next person can choose a new gift or steal the first gift by saying.
I’m June and "it is a dirty rotten shame Suzy, I want the scarf". Then Suzy chooses another gift. The third person can steal either gift or pick a new one, and so on until everyone has a gift. You cannot steal back something that was previously stolen from you.
An online search will yield dozens of hits for office Christmas party games including Christmas trivia. The web can also be a source of cheap supplies and decorations.