Sleightholme, whose father Jon also played for Northampton and England, first joined his hometown club at the age of 13.
He made his first-team debut in a cup game against Bristol in October 2018 and scored a try after only 15 seconds on his Premiership debut against Wasps the following month.
“Keeping that core group together and making sure we continue to push forwards and improve with them is something we’re always working on,” said director of rugby Phil Dowson.
“Ollie’s definitely one of those guys who has that out-and-out quality already, but he’s got the potential to keep getting better as well.
“His strike-rate is remarkable, and it’s hard sometimes as a coach here, because everyone in that backline is playing so well and it’s extremely competitive.
“But Ollie’s consistency is clear to see from his stats and that’s starting to get recognised at international level too which we’re delighted about.”