Trailing 14-10 with under a minute on the clock, the Mustangs had one final attacking set.

Lock Harold Thompson received the ball two passes wide of the ruck and stepped three defenders, got his body low and dived over to level the scores.

In gale force winds, the ball fell off the tee on two occasions before Kaine Parkinson calmly slotted the conversion which sent the Mustangs into the decider and jubilation.

"It was just elation and celebration after the conversion went over, it never looked like missing despite the wind," Mustangs Under-18.5s co-coach Reuben Jones told the Argus.

The Mustangs shot out to an early 10-0 lead before repeated errors and infringements opened the door for the Pirates to build momentum.

Jones was disappointed his team went away from the game plan which had been working for the opening 20-minutes.

"We had been getting our outside backs involved early and were moving the Old Bar forwards around," he said.

"But then we went to too much one out hit ups and that plays directly into their hands."

The Pirates reduced the deficit to 10-6 before half-time and levelled the scores after the break.

Old Bar controlled the majority of the possession but the Mustangs' defensive line held firm until the final 5-minutes.

An Old Bar prop managed to bust off a would-be defender and dove over to give his side their first lead of the match.

From the resulting restart, the Mustangs regained possession and on the back of a repeat set for a penalty, Thompson scored the winner.

"That win comes down to our goal line defence. They had all the possession and territory but the boys defended their hearts out," Jones said.

"We were able to win ugly and I think that says a lot about this team.

"Hopefully we can play out best in a fortnight."

The Mustangs Under-18.5s will have a week off before they take on the winner of the preliminary final on Sunday August 25.