Farhat's long overdue third test century enabled him to become only the fourth Pakistani batsmen to carry his bat through an innings. His 117 spared the tourists from total capitulation after retirement-bound O'Brien opened his farewell test with a four-wicket burst that had them reeling at 51 for five before lunch.
Thanks to Farhat's charmed 286 minutes at the crease - and valuable assistance from the tail - Pakistan recovered to post 223 on a pitch not as treacherous as the scorecard indicated.
In the 19 overs available before bad light stopped play, New Zealand reached a soothing 47 without loss, their highest opening partnership of the series.
Like Farhat, Tim McIntosh (31) and debutant BJ Watling (13) both enjoyed reprieves. McIntosh successfully employed the umpire decision review process after being adjudged leg before wicket to Mohammad Asif by Billy Doctrove when yet to score . Then there was a suspicion Watling might have been caught at the wicket on 10 but Pakistan did not refer Doctrove's rejected appeal to third umpire Simon Taufel.
McIntosh and Watling resume eating into a 176-run deficit tomorrow. Pakistan's decision to bat on a wicket predicted to offer the seamers greater than usual assistance was under scrutiny once Tim Southee celebrated his 21st birthday by uprooting Salman Butt's middle stump.
O'Brien removed the recalled Faisal Iqbal, then dislodged Pakistan's batting axis - Mohammad Yousuf and Umar Akmal - before they could record a run between them.
Yousuf edged a cramped cut to McIntosh at second slip to end an uncomfortable 14-ball stay, while Akmal gave Martin Guptill catching practice in the gully instead of leaving a short ball or plugging it through point.
Four balls later O'Brien had Misbah-ul-Haq caught at the wicket - the first of Brendon McCullum's five catches - to give O'Brien a dream analysis of four for three from five overs.
The 33-year-old's subsequent spells went unrewarded as Daryl Tuffey (4-52) mopped up a tail that dragged Pakistan from 90 for six to a semi-respectable total.
Farhat was at the core of the rebuild, although he showed the same capacity to self destruct.
Fortunately for the 27-year-old, the top edges and ungainly slogs fell harmlessly - he offered only one half chance on 105 when an unplanned deflection dropped just short of Martin Guptill in the gully.
Farhat, an intermittent selection since his debut in 2001, completed his first century since making 101 against India at Lahore in April 2004.
Never afraid to ride his luck, Farhat's ton contained a dozen boundaries and a six and came from 138 balls. His 10th boundary summed up his good fortune when a planned swing through mid wicket crossed the rope third man to take him to 83.
Farhat ultimately struck 14 boundaries in his 169-ball stay and, when Danish Kaneria became Tuffey and McCullum's last victim midway through the 65th over, became the first Pakistani opener to bat through a completed innings since Saeed Anwar's unbeaten 188 against India at Kolkata in 1999.
Although Farhat contributed more than half the runs, he received some valuable support from bowlers who put their batsmen's input to shame.
Mohammad Aamer dug in with 23 from 82 balls as the teenager added 69 for the seventh wicket. Umar Gul chipped in with a sprightly 24 while Kaneria's quartet of boundaries lifted the score over 200.
O'Brien was denied a deserved five-wicket bag after Tuffey, on a hat-trick when Aamer and Asif were gloved by McCullum in successive balls, became the last of the four-pronged pace attack to taste success.
But O'Brien's four for 35 from 15 overs was easily the stand out analysis.
Shane Bond orchestrated the perfect comeback to test cricket in Dunedin and O'Brien has one more opportunity to finish up in style.
