It must have been an intimidating proposition for an unheralded amateur to take on these and other greats of the professional game. As far as they were concerned, this Irish upstart was "the amateur" who would appear in their midst for a year, maybe two, before disappearing again. But they were mistaken, just as those who came after them, golfers of the stature of Neil Coles, Noel Ratcliffe, Eddie Polland, Ian Stanley, Terry Gale and so on got it wrong. O'Sullivan was there for the long haul and earned their grudging admiration through hard work; that in turn produced a remarkable improvement in his golf game and a very agreeable standard of living.
Given the nature of seniors golf, new faces are coming on the scene all the time but Denis isn't fazed in the slightest that people like Greg Norman, Sam Torrance, Nick Faldo, Bernhard Langer, Seve Ballesteros and Ian Woosnam, to name but a few, have either turned 50 or are approaching the half-century.
"My game is in good shape and my goal for 2006 is to finish in the top five in the order of merit. I won't be worrying about what anybody else is doing," he said.
Having successfully come through the Seniors Tour School in 1997, Denis quickly adjusted to the new environment in which he found himself. He played 16 tournaments in 1998 and came 9th in the order of merit with total earnings of €76,986. It was an outstanding achievement by any standards.
'Second season syndrome' set in the following year and he slumped to 33rd in the order of merit with €32,598, in truth no more than a break-even situation. However, everything settled back into place in 2000 when he captured two major tournaments towards the end of the year, the Senior Tournament of Champions and the Seniors Tour Championship which shot him up to third in the rankings with the handsome total of €156,966 to his credit.
Two further victories followed in 2001 the Palmerston Trophy in Berlin and the Scandinavian International and he duly finished 12th in the order of merit with €165,920. Denis captured his fifth title in 2002, the Tunisian Seniors Open, and was back up to 9th in the money list with €125,362. Though 2003 and 2004 were relatively quiet with no tournament wins, his consistency was extremely impressive as he came in 8th in 2003 with a stroke average of 71.04, which accumulated €130,427.
However, he was down to 34th in 2004 as the stroke average crept up to 72.94 and the prize money dipped to €64,433.
The Doubting Thomas' were quickly into action, predicting that age was catching up on the Corkman and that at 56 his best days were behind. Not alone that but the European Seniors Tour was becoming a tougher and tougher place in which to do business, given the annual arrival of some of the game's finest. Sam Torrance of Scotland led the order of merit in 2005, and Eduardo Romero of Argentina was another formidable operator to pass the 50 mark. Put it all together and Denis might have been in trouble.
Only too well aware of the need to set the bar still higher, he practised religiously in the off-season and came out at the beginning of the year focused on proving that he still had every right to compete in an ever increasingly competitive arena.
The first tournament on the programme was the Barbados Open over the scenic and testing 6,814-yard Robert Trent Jones Junior Royal Westmoreland course, and O'Sullivan romped home by three shots after a ten under par aggregate of 206. He went on to perform consistently throughout the year before finally finishing in ninth place in the order of merit (his fifth top ten in eight years) with €149,867, bringing his career earnings to €896,618. And, of course, he'll be back to defend in Barbados on March 1-3.
"I'm quite happy at the way 2005 went off because it's getting a lot tougher out there," says O'Sullivan, who will be 58 in February.
"A lot of younger guys come on to the tour every year and obviously the standard is rising all the time. For instance, the really big guns, Langer, Faldo, Seve, Woosie and so on, will all be turning 50 over the next couple of years. In fact, Faldo becomes eligible for the British Seniors on the week before it takes place at Muirfield where he won two of his Open Championships in 2007."
It's becoming more and more difficult to obtain and retain a card on the Seniors circuit but true to form, O'Sullivan's not complaining. He fully appreciates how good the Seniors Tour has been to him although, of course, the reverse is very much the case as well. He and his partner Paula aren't long back from a week in beautiful Mauritius, where he took on and beat a number of players from the regular European Tour. For the second successive year, his friendship with Old Head captain Tom Browne will enable him to spend five weeks or so playing three courses in the Naples area of Florida, where he also finds the practice facilities as good as one could imagine. Then there's the news that the AIB Irish Seniors Open is coming to Fota Island on the first weekend in June. What a mouthwatering prospect that presents for a proud son of Cork.
"Believe it or not but I'm still trying to improve," he insists. "My putting was not as good as it should have been in 2005, so that's an area I'll be working on over the next few years. It's good for the Tour that all these new guys (including my old friend Jimmy Heggarty) are coming on stream all the time. And it's also good for the rest of us because we know we'll be left behind if we sit on our hands and don't work at getting better. It's a big challenge but one I'm very much looking forward to."