Can I sue an office because one of their employees assaulted me?

It depends. What kind of office? Did the assault occur in the office? Was the assailant working at the time? Was the contact between you and the assailant in any way related to the business you had with the office? To your knowledge, was is the first time this employee assaulted anyone? At the time of the assault, was the assailant's supervisor aware of what was happening? Was is reasonable to expect that the management would prevent the assault? And so on... The best is to talk to an attorney and give him/her all the details.