The company said it lost $10.54m on a net basis from 1 to 31 May.
Revenue was $81.42m, but operating expenses were $82.28m.
Reorganisation costs wiped $11.9m off the bottom line.
In April, OSG posted a loss of $6.25m.
Restructuring US tanker owner OSG has revealed a bigger loss in May, as its total deficit since filing for bankruptcy in November 2012 reached $969m.
The company said it lost $10.54m on a net basis from 1 to 31 May.
Revenue was $81.42m, but operating expenses were $82.28m.
Reorganisation costs wiped $11.9m off the bottom line.
In April, OSG posted a loss of $6.25m.