The extra resource was found in an area not under production in the current drilling campaign.
The Ocean Patriot rig being used is not deep enough to drill as part of an exploration program for the Galoc field.
Otto will push back exploration drilling until after it finishes its program and when it can secure a suitable rig.
Otto chief executive officer Gregor McNab said the Galoc field was continuing to perform beyond expectation.
"We look forward to progressing drilling of a Galoc exploration well at the preferred drilling location as soon as a suitable rig can be secured," he said.
"In the meantime, Otto Energy looks forward to safe execution of the Galoc Phase 2 project, drilling of the Duhat-2 well and acquisition of seismic in our two Tanzanian blocks over the coming months."
The Galoc field has produced 10 MMbbls of oil with an extra 11 MMbbls 1P to be developed through Phase 1 infrastructure and Phase 2 drilling.