He worked as an accountant for Ernst and Young for about a year, before enlisting as a member of the New York City Fire Department.
O'Berg was killed at the age of 28 on September 11, 2001, when the World Trade Center towers fell as a result of the September 11, 2001 Terrorist Attacks. O'Berg's father (also Dennis O'Berg, and a firefighter) was known to be onsite at ground zero for weeks after the disaster, in the hopes of finding the remains of his son; he retired the day of (and as a result of) the terrorist attacks. Whether Dennis O'Berg the younger's remains were ultimately found is not known as of this writing.
A memorial service for him, with honors, was held on June 28, 2002.