The 27-year-old front-row will join the club at the end of the season, with O'Shea insisting that the prop will "add greatly" to the exciting set-up.
"While we were disappointed to lose James Johnston we recognise the need to have an experienced player to come in to supplement the existing tight head talent we have at the club in Will Collier and Kyle Sinckler," he told reporters.
"With that in mind we are very fortunate to be able to bring in a player of Paul's ability. Not only is Paul talented, at his age he will only get better.
"He comes from a fabulous club and has been coached by one of the best forwards coaches in the country so I have no doubt he will add greatly to what we have at Harlequins. We look forward to welcoming him to The Stoop at the start of next season."
Doran-Jones added: "I'm delighted to have the opportunity to play at the best club in the Premiership. The team play with smiles on their faces, a sure sign that they enjoy their style of rugby.
"I'm looking forward to being part of that, to improving my game and to working with the coaches and players to contribute to further success for Harlequins. In the meantime, I'm committed to doing my best for everyone at Northampton Saints and helping the club to a great finish to the season."
Doran-Jones has four England caps to his name.