Gardner-Webb's Tyler Strange made a 3-pointer to tie the game 57-57 with 43 seconds left in regulation.
Hartley hit a 3-pointer for a 62-59 lead in the extra period, and the Bulldogs knocked down 7 of 8 free throws in overtime to claim the victory.
Tashan Newsome scored 14 points and Strange 11 for the Bulldogs (19-11, 10-5 Big South Conference), who have won six straight.
Saah Nimley had 15 points and Paul Gombwer 14 for the Buccaneers (16-11, 11-4), who lost to the Bulldogs 74-73 in overtime in January. Charleston Southern is tied with High Point, which lost to Radford 63-58, in the conference.
Gardner-Webb made 11 of 28 3-pointers to Charleston Southern's 5 of 17.
Guard Naz Mitrou-Long, whose 2015-16 season ended in December due to hip problems, has been granted a hardship waiver for the coming season, Iowa State said.
As the Sooners look to life without superstar Buddy Hield and his sidekicks, it's clear their unseasoned players will need to grow up fast.
The Central Intercollegiate Athletic Association says it won't move its headquarters, its basketball tournament or other conference championships from North Carolina, despite