OBERON has sprung a round one upset, knocking off Group 10 premiers Orange CYMS 20-18 in front of a raucous home crowd in Oberon.

The Tigers fought back from 18-8 down in the second half, scoring late tries through Trent Rose and a second-half double to Tyler Hughes to steam roll their more fancied opponents, former St George-Illawarra, Melbourne and Manly prop George Rose laying the foundation for the win.

A huge day for Oberon that began with juniors in the morning and crescendoed with all four senior grades in the afternoon, a massive crowd of nearly 1300 people packed into the Oberon Sportsground to see their new-look premier league team record club's first top grade win since 2014.

Orange CYMS captain-coach Mick Sullivan said the most disappointing part of the loss was his side's second-half performance.

"We just couldn't hold them out," he said.

"It's something we need to look at. Losing 12-0 late in that second half after leaking 30-odd points in the final of the Bathurst knock-out, it's not good enough and we don't want that becoming a habit.

"They're a big side, rolled through the middle with George and worked off the back of that.

"Then when their forwards got tired their wingers and centres came in and were outstanding."

The Dave Scott medallist in last year's grand final win with CYMS, Oberon's gun recruit Trent Rose believes his side's win was based on "guts an determination from all the boys".

"There was a stage at the match where it could have went either way ... the score could have blown out or we could dig deep and fight back. We all had a good talk and look at ourselves after CYMS' last try and the tide changed," Rose said.

"And the support from the crowd helped massively to help get us over the line.

"They never gave up on us so it was only right we didn't give up on them and ourselves."

CYMS began well, scoring the game's first two tries to lead 10-0 after Sam Hill darted over and winger Dylan Wood scored out wide, Ben McAlpine landing one of the conversions to shoot the premiers out to an early lead.

Returning backrower Riccie Arriola scored at the half-hour mark to drag Oberon back into the game, with Trent Rose's successful conversion followed by a penalty goal right on half-time to make the score 10-8.

Starting in similar fashion as they did in the first half, CYMS crossed for two early second half tries through McAlpine and Brock McGarity to make visitors' advantage 10 points.

But a try to Trent Rose soon after turned the tide and a Hughes' brace then shot Oberon to a 20-18 lead and ultimately the win.

"Unreal to have the first-up win after going about 18 months without a victory ... to get a first-up win against the reigning premiers and benchmark team for this season is an amazing effort," Rose added.

Sullivan believes CYMS showed enough to be able to hit back in a fortnight's time when the green and golds hosts Lithgow in a rematch of last year's grand final.

"Our shape was good in patches. I was pretty pleased with that," Sullivan said.

"They just went through our middle, which was a bit disappointing."

OBERON TIGERS 20 (Tyler Hughes 2, Trent Rose, Riccie Arriola tries; Trent Rose 2 goals) def ORANGE CYMS 18 (Ben McAlpine, Brock McGarity, Dylan Wood, Sam Hill tries; Ben McAlpine goal)