The 20-year-old came through qualifying and won eight matches to claim her first tour title at the Wimbledon warm-up event. She also earned a wild card to compete at the All England Club next week.
"It was like this came out of nowhere," said Oudin, who didn't expect to win her first title on grass. "I'd been working away for a long time, and I was waiting for something to happen and it didn't. Until now, that is."
Oudin, who has been recovering from an injury, become the first player ranked outside of the top 200 to capture a main tour title since Kim Clijsters won the US Open in 2009.
Oudin was eliminated in the second round of French Open last month by Sara Errani, who reached the final. Oudin was a 2009 U.S. Open quarterfinalist, but has struggled since that breakthrough.
She also beat Jankovic before -- during their first and only previous meeting in Wimbledon three years ago.
On Monday, Oudin used a strong forehand and slice on the grass and moved well despite playing seven matches to reach the final.
Jankovic struggled with concentration and movement. She appeared tired at times after the 4-hour, five-set semifinal match against Zheng Jie of China the previous day.
Both players competed in two matches Sunday because of rain delays.