O’Brien repeated his feat from the previous round, at Hull KR, with the 83rd-minute winner after a dramatic quarter-final with Salford Red Devils, at Emerald Headingley, had been locked at 18-18 following normal time.
The half-back/full-back was sin-binned in the closing stages, then came back and conceded a penalty which led to Salford’s equalising converted try on the last play of normal time.
“I think I was getting the sack if that didn’t go over,” O’Brien said of the winning kick against his former club.
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“It is a bit of a silly sin-binning and then I give a penalty away for them to march us down the field and obviously Lee Mossop goes over.
“I had to step up and that went over and I am delighted to be in the semi’.”
In his pre-match press briefing, coach Daryl Powell said his side had been practicing drop goals after their flurry of misses against Hull KR.
“Me, Danny [Richardson] and Truey [Jake Trueman] are knocking at least 20 over each week now,” O’Brien confirmed.
“Thankfully that was first time, not like the KR one that took us 20 minutes, so I am buzzing to be in the semi-final against Warrington.”
The tie on June 5 is another meeting with one of O’Brien’s former sides and he predicted it will be “a cracker”.
He said: “Hopefully I can get selected for that, against my hometown club.
“The prize is the Challenge Cup final and I’ve not played in one yet so I will be itching to get there.”
Tigers used all three of their half-backs, with O’Brien coming off the bench in the second half to replace Richardson, who started alongside Trueman.
Powell said: “I was joking to Gaz after the game, we just need to keep him on the field.
“He has had two injuries and a sin-binning, so keep him on the field, get a bit of consistency in terms of our half-backs and we will be good.”
Despite his relief at the win, Powell accepts his men will need to improve to reach Wembley for the first time since their 2014 loss to Leeds Rhinos.
They trailed twice, 6-0 and 12-6 and narrowly failed to hold on in normal time after going ahead on the hour.
But, having been reduced to 11 men briefly in the final stages – when Salford were throwing everything at them to snatch an equaliser – the coach could not have been prouder of their effort.
“We had to work so hard, with two sin-binnings and them getting us right at the end,” he reflected.
“I thought we did really well to battle through some difficult periods.
“The amount of effort and commitment from the boys, down to 12 and then down to 11, that is very difficult, to defend against a team with the ability of Salford.
“It was massive to keep them out, an immense effort and it is fantastic to be in the semi-finals.
“It is a good opportunity and exciting for us as a club and a team.”
O’Brien was the match-winner, but the day’s top performance came from Man of Steel Paul McShane.
He charged down a drop goal attempt by Kevin Brown at the end of the first set in extra-time, managed to gather the ball and set up field position for O’Brien’s kick.
That came after his pass sent Dan Smith in for Castleford’s first try and he made a break and offload for Michael Shenton’s second-half score.
Powell described McShane’s extra-time play as “class” and enthused: “He is playing great.
“He loves his rugby league and he’s a little general in the team.
“I just want him to run a little bit more, because when he does he is phenomenal.”
The coach confirmed Tigers’ veteran forward Grant Millington faces a spell on the sidelines after suffering a suspected medial knee ligament injury.
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