The six-year-old, wearing the colours of the Supreme Horse Racing Club and Kenneth Sharp, got the upper hand from the final fence.
The 15-8 shot was completing a notable across-the-card double for Willie Mullins after the victory of True Self at Bath, as well as giving Ruby Walsh a second winner since he returned from injury on Sunday.
Ornua made a bold bid to lead all the way.
But Cadmium took over at the last, reversing Roscommon form with Ornua – who stuck on well for second place, five lengths adrift of the tidy winner.
“There is a fair chance that I’m blowing harder than the horse!” said Walsh.
“He did it well. It was hard to see how he would reverse the form with Ornua – but I suppose a more galloping track, the two furlongs extra and the 3lb swung it in our favour today.
“That’s twice he’s won here. He’s a novice for another six weeks, so you’d imagine he’ll come back here in a month’s time for the Craddockstown all being well.”
On his return to action, Walsh said: “I feel like coughing up half a lung at this minute in time – but other than that, we’re all right.”