He inherited a team that was something of a shambles after Deano humiliated the club with Bloodgate. Quins had good players, but the leadership was broken by Bloodgate, and Quins were a nuggety team who lacked a bit of attacking edge.
COS oversaw a complete overhaul of the Quins style of play, promoted more youth, and took a decent team and turned it into a title-winning one, playing a fabulous brand of rugby and ensuring we kept hold of pretty much every important player we had.
It just isn't true to say he "kept the ship steady". Under Deano, we had one good season - the Bloodgate season of 2008/09 - that ended with us getting absolutely hammered (0-17!) in the playoff semi-final, at home, against London Irish. We also lost some important players after that, particularly Mike Ross who'd been a huge part of our scrum dominance. The team essentially fell apart and were pretty useless. COS took over in March 2010.
After that... it wasn't perfect but in his first season we beat Munster away on the way to winning silverware in the Amlin, which included an epic final showcasing our never-say-die attitude and the talent in the team. In 2011-12 we won the league playing an awesome brand of fast-paced, offloading rugby. In 2012-13 we made the playoffs again and won the Anglo-Welsh cup. In 2013-14 we made the playoffs again.
It didn't last with COS but he did some wonderful stuff for the club: he really promoted a lot of youth, he committed us to a brilliant style of rugby, he turned things around after the disaster of Bloodgate, and he won us multiple trophies.
You only have to look at how bad things were immediately post-Deano, and how mediocre we were before Deano's final season, to realise what he achieved at the club.
Both guys deserve credit for some canny signings and allowing the youth to prosper. But I feel Deano gets a bit too much credit for things that happened years after his departure.
These are the teams for the biggest games (Bloodgate match and the Premiership final):
Harlequins: Brown, Strettle, Tiesi, Turner-Hall, Monye; N Evans, Care; Jones, Botha, Ross, Percival, Robson, Robshaw, Skinner (capt), Easter.
Replacements: Malone for N Evans (47), J Evans for Percival (69), Williams for Malone (69). Not Used: Fuga, Lambert, Guest, Gomarsall.
Harlequins: Brown; Williams, Lowe, Turner-Hall, Monye; Evans, Care; Marler, Gray, Johnston, Kohn, Robson, Fa'asavalu, Robshaw, Easter.
Replacements: Buchanan, Lambert, Collier, Vallejos, Guest, Dickson, Clegg, Hopper.
That's no Strettle, Tiesi, Jones, Botha, Ross, Percival, Skinner (plus Malone, Fuga, Gomersall). Kohn was injured for the Leinster game IIRC.