Phillips with his third goal of the campaign and O'Connor sentenced injury and suspension hit Barnsley to their fourth successive defeat.
Phillips and O'Connor had also been on target in the 2-1 win at Southampton and their partnership was too much for Barnsley as Birmingham extended their 100 per cent start to the season.
Barnsley were full of endeavour but lacked the cutting edge that Birmingham possessed.
Their best chance fell to skipper Brian Howard in the 57th minute, but he couldn't beat goalkeeper Maik Taylor.
Phillips had gone close to breaking the deadlock in the eighth minute when a near post header flashed just wide.
But he couldn't miss five minutes later when Barnsley failed to fully clear a corner after Luke Steele had beaten away a drive from Stuart Parnaby.
James McFadden, who had seen a fifth minute overhead kick flash inches too high, fed the loose ball to Parnaby whose pinpoint left-wing cross was headed home by Phillips from six yards.
McFadden caused chaos down the left wing and Barnsley were fortunate not fall further behind after 17 minutes when the Scotsman held off Dennis Souza only to fail to hit the target from 12 yards.
Failing to turn all their early possession into more goals almost proved to be costly for Birmingham five minutes before half-time.
In a rare threat from Barnsley, Martin Devaney slid the ball into the path of Diego Leon and his 25 yards drive beat Taylor only to crash off the bar.
That escape sparked Birmingham back into life and Barnsley's challenge was effectively ended in the final minute of the half.
Steele failed to deal with a left-wing cross from Quincy Owusu-Abeyie and O'Connor was on hand to head home at the far post.
With nothing to lose, Barnsley manager Simon Davey sent on striker Kayode Odejayi in place of midfielder Bobby Hassell at half-time and the Oakwell outfit did carry more of a threat after that move.
It was Odejayi who set up Howard with a neat knock down but Taylor was well placed turn the ball away with his legs.
Devaney then fired over the bar from 10 yards which ended Barnsley's hopes of a revival.
Birmingham were denied a more conclusive win by two late saves from Steele, who frustrated McFadden and Gary McSheffrey.