O'Shea's joy is understandable, given the impact New Zealand fly-half Evans has made since arriving at Quins from Otago in 2008.
Among the finest overseas signings made by any Premiership club in the professional era, Evans has amassed almost 1,200 points for Quins and was a driving force behind last season's league title success.
"We have started something here and Nick wants to see it through," said O'Shea, ahead of Saturday's home clash against third-placed Gloucester.
"His decision to stay is a great early Christmas present for all Harlequins supporters.
"The Aviva Premiership wants the best players playing in it and Nick is one of the very best. He contributes so much to this club both on and off the field, and he is the perfect role model for all young players."
Gloucester will arrive at the Twickenham Stoop on the back of an eight-game unbeaten run in all competitions, but rugby director Nigel Davies has retained a level-headed approach.
"We are not going to be making any big predictions about how good we can be or where we can go because it's still very early days for us," Davies said.
"We're just going about our business pretty quietly and getting on with things, and there is no need for any big pronouncements or anything like that.
"We are delighted with the win over Leicester (last Saturday), but it's just one game and now we have got another massive challenge ahead this weekend."
Elsewhere tomorrow, fierce rivals Leicester and Northampton meet at Welford Road when both sides will be shorn of several England players, while Exeter host Worcester.
And Sale Sharks will hope that the John Mitchell effect can rub off on them when they aim to end their demoralising run of seven successive Premiership defeats tonight.
Sale, eight points adrift at the Premiership basement, entertain 11th-placed London Irish at Salford City Stadium.
The game comes after a difficult few days that saw Bryan Redpath removed as Sharks rugby director and appointed to a new role of head coach.
Club chief executive Steve Diamond will be in charge of team matters until the end of this season, but former Sale boss Mitchell has helped with preparations this week before returning to South Africa, where he is currently engaged in a legal dispute with the Johannesburg-based Golden Lions.
"He (Mitchell) has come as a consultant just to give a helicopter view of how we do things and how we can smarten things up," Diamond said.
"He has worked extensively with Bryan and myself, and it seems to be going well."
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