Quins director of rugby O'Shea kept faith with Care, who has been questioned and bailed by West Yorkshire police over an alleged sexual assault on a woman in Leeds earlier this month. He denies the allegation.
Care, previously dropped from the England squad following a drink driving conviction, has already accepted a police caution for being drunk and disorderly that night.
O'Shea said: "I was never going to leave him out.
"Danny's made his mistakes, they are well documented. He's learning from his mistakes.
"What he has to understand is that he will be an easy target and we are absolutely supporting him in this case."
O'Shea insisted that the club are supporting the player, adding: "There's plenty of stuff we've put around Danny that he's working on, both from technical scrum-half play to lifestyle. We are absolutely behind him."
O'Shea also defended Maurie Fa'asavalu, Quins' Samoa World Cup flanker.
The former St Helens rugby league player scored the only try of the match before being yellow-carded by referee Wayne Barnes for an illegal challenge on Bath's teenage fly-half Tom Heathcote.
Bath rugby director Sir Ian McGeechan, who revealed that Heathcote suffered a whiplash injury when Fa'asavalu charged into him without using his arms in the tackle, said: "I hope the citing officer is looking after it.
"He (Heathcote) is 20. We've got to look after these boys."
But O'Shea said: "My immediate thoughts are to have a long, hard look at it.
"I haven't seen it in slow motion but he comes in to make a dominant hit, he does try to get his arms out and around but Tom Heathcote is falling.
"The referee saw what happened and deemed it a yellow card.
"People who know Mo, both from St Helens days and from here, know he's not a loose cannon going round the pitch. He's an incredibly physical bloke but he's not a dangerous player."
Victory restored Harlequins to the top of the Premiership less than 24 hours after reigning champions Saracens deposed them with a 45-9 win at Sale.
But Bath pushed them all the way. Only a knock-on by winger Olly Woodburn before he touched down denied them a 54th-minute try when they were 6-3 ahead.
McGeechan added: "I think we could have won it on another day. A try then would probably have seen us away."
McGeechan was also unhappy with the yellow card shown to Bath's replacement full-back Jack Cuthbert, judged to have challenged Mike Brown in the air as the Quins' full-back collected a high kick.
McGeechan, who claimed Brown's feet had touched the floor before the tackle, added: "A couple of decisions were against us. His feet were on the ground but the referee's got to make an instant decision.
"The critical ones like that cost us, Quins scored the points when Jack went off so it wasn't the best timing in trying to keep the momentum in our game.
"Quins are a good side - that's why they are top of the league - but I thought we looked a good side today."
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