The keeper, who is currently injured, will join the League One club for the remainder of the season, with the hope that he will get regular game time when he is fit.
It is a straight loan deal, with the keeper a few weeks away from returning to action, after he picked up an ankle injury at the start of January, prompting the Cherries to bring Fraser Forster into the club on a short-term deal as cover.
Dennis started the campaign as Bournemouth’s second choice keeper, behind summer signing Djordje Petrovic.
Will Dennis. (Image: Richard Crease)
The stopper joined the club’s development squad at the age of 15 in 2016, before he turned professional 18 months later.
He was sent on loan spells to Gurnsey, Weymouth, Wealdstone, Slough Town and Klimarnock.
In Scotland, he featured 44 times in all competitions for the Premiership side in the 23/24 campaign, helping them to a fourth-placed finish in the league and to the quarter finals of both domestic cups.
The 25-year-old has played once for the Cherries, conceding one goal in a 4-1 win over Oldham Athletic in the FA Cup.
He signed a new deal with the club in February of last year, which runs until 2028.
But, with Dennis' injury and the arrival of veteran Forster and Lazio loanee Christos Mandas in January, he has been sent on loan for the rest of the season.
Orient currently sit 18th in League One, with 32 points from their 29 games.