Registration problems prevented the Sons from completing the signing of the 21-year-old Ghanaian, previously of Europa Point in Gibraltar.
Heffernan, the 34-year-old Irishman, had been released by Championship rivals Queen of the South in January.
Sons boss Stevie Aitken said: "I'm really happy, especially after the disappointment of losing Sebastian."
Heffernan, who had joined Queens after leaving Dundee in August, has only made seven appearances, scoring once, this season.
Meanwhile, Dumbarton have invited Osei-Obengo back to train with the club in the summer.
Dumbarton have signed experienced striker Paul Heffernan after a deal for Sebastian Osei-Obengo fell through.