Both sides went into the game at Fratton Park in dire need of three points, with Pompey having won just once in their previous 10, and Blackburn having drawn three on the bounce to continue to languish just above the bottom three.
The visitors were boosted by a first start in two months for skipper Todd Cantwell following injury, but both sides lacked invention and chance creation in a lacklustre first half, with Terry Devlin’s long-range shot being the only attempt in the opening half-hour to trouble either goalkeeper.
Ohashi changed all that as he led a Blackburn counter-attack from a Sidnei Tavares pass and with Andre Dozzell and Devlin giving him too much space, he carved between the two defenders and placed his shot in the far corner for his fifth goal of the season.
Pompey stepped up the pressure in the second half and only some last-ditch defending prevented them from levelling as Rovers keeper Aynsley Pears twice blocked Chaplin efforts, sandwiching a Lang shot which was stopped on the line by Yuri Ribeiro in a remarkable action sequence.
The pressure was clearly mounting and shortly afterwards Lang struck, getting on the end of Josh Murphy’s superb whipped cross to finish on the half-volley at the far post.
Blackburn wilted and Pompey finished strongly, and a side which has suffered from set-pieces this season actually won the game via that route, as Chaplin’s corner saw Bowat rise highest to head home and spark a big Pompey celebration.