O'Sullivan's 79th-minute second was crucially converted by fly-half David Connellan as John McKee's men moved out of relegation trouble with a 17-16 success.
Shane Whelan had kicked the second-from-bottom Dubliners - without a win since January - into an early lead, and the scrum-half also converted lock Alan Maher's touchdown on the half-hour for a 10-5 break buffer.
O'Sullivan was Buccs' initial scorer - notching the westerners' 300th league try in all, in reply.
On the restart, Lansdowne looked to take a hold on proceedings but although Whelan successful with two more penalty attempts, Connacht lock Pieter Myburgh was allowed in for a second Buccs try.
Centre O'Sullivan made Lansdowne pay further for allowing the visitors back into it, as he ended a sizzling 40-metre run, dummying past Lansdowne full-back Johnny Murphy, to set up a grandstand finish.
Connellan arched over a brilliant touchline conversion, and with both Whelan and Andy Tallon wayward with two drop goal attempts in injury time, Buccs held on to move up to eighth.
AIB League Results: Saturday, March 5
Lansdowne 16 Buccaneers 17, Lansdowne RoadScorers: Lansdowne: Try: Alan Maher; Con: Shane Whelan; Pens: Whelan 3Buccaneers: Tries: Paddy O'Sullivan 2, Pieter Myburgh; Con: David Connellan
Ballynahinch 6 Dolphin 26, Ballymacarn ParkDe La Salle Palmerston 0 Young Munster 16, Kilternan
Ballina 10 Ards 38, Heffernan ParkBanbridge 12 Suttonians 25, Rifle ParkBruff 34 Skerries 15, Kilballyowen ParkCity of Derry 22 Wanderers 12, Judge's RoadConnemara 3 Sunday's Well 13, Monastery FieldCorinthians 13 Queen's University 8, Corinthian ParkInstonians 41 Old Wesley 6, Shaws BridgePortadown 8 Clonakilty 33, Chambers Park