Down opened their All-Ireland round-robin series campaign by easing past Clare in Ennis while Cavan shocked Mayo in Castlebar.
The Mourne County produced a blistering first-half display to lead by 14 points at the break before staying clear to win by 17 points in their Group Three opener.
But Cavan pulled off a huge shock result in Group One by sinking the Connacht finalists 1-17 to 1-14.
The Breffni men led 0-9 to 0-7 at the break with Mayo failing to score from play until the 32nd minute before losing defender David Neary to a black card.
Cavan took full advantage of their numerical advantage early in the second half with 1-3 in the opening five minutes with Gerard Smith scoring his side’s goal.
O’Reilly kicked the Ulster side into the lead in the 39th minute and it was an advantage they refused to relinquish.
Cian Madden and Stephen Coen exchanged scores as Cavan managed to keep their noses in front.
Aidan O’Shea hit the net for the home side with the final action of the game but it was Raymond Galligan’s side who claimed a shock three point win opening up group one.
Cavan host Ulster champions Donegal in a fortnight while Mayo will travel to Omagh to take on Tyrone looking to pick up their first points.
Meanwhile, Down got off to a flying start in Ennis, surging into a 1-3 to 0-0 lead after five minutes with John McGeough firing to the net.
Danny Magill kicked six first-half scores, which eased Conor Laverty’s side well ahead.
They led 2-12 to 0-5 three minutes before the break as Daniel Guinness swept through to raise a green flag.
Two minutes later, Aaron Griffin hit the net for the Munster finalists but they trailed 2-17 to 1-6 at the interval.
In the second half, it was the Ulster side who held firm, adding a third goal through Caolan Mooney. Pat Havern kept the scoreboard ticking over, too, as he finished on nine points.
Having begun their campaign in style, Down will welcome Leinster champions Louth to Pairc Esler in a fortnight’s time.
