The underwriters will now purchase 4.7 million flow-through units priced at $4.30 and 6.4 million units priced $2.35.
Each unit will comprise a share and half a two-year warrant priced at $3.25.
Proceeds will be used for exploration of the company's projects in the Val d'Or region of Quebec.
O3 is working on a preliminary economic assessment for its Marban project while expanding the mineral resources at Alpha via a 25,000m drill programme and at the East Cadillac property via a 15,000m programme.
Shares in O3 Mining are trading at $2.52, valuing the company at $119 million.