The Nigerian opened the scoring in the 18th minute, but the Ligue 1 outfit levelled through Lepaul before half-time.
Gabriel Sara restored the lead for the Turkish team shortly after the restart, only for Lepaul to equalise again before completing his hat-trick from the penalty spot to put Rennes 3-2 ahead.
Galatasaray rescued a draw deep into stoppage time when Barış Alper Yılmaz converted from the spot after Osimhen was brought down inside the penalty area.
The result provides another useful test for Galatasaray ahead of the new campaign.
They will continue their preparations against Spanish side Villarreal at RAMS Park on Saturday, August 8.
