This depends on what you want to do. If you are making a solution, how many milligrams per milliliter do you want the solution to be (or g/L as the case may be)? If you have a liquid and were told to...
The chemist (or anyone who cares to simply look it up) knows that the density of oxygen is about 1.429 g/liter at STP. That's 1429 mg/1000 ml or 1.428 mg/ml for your answer.