Measure the concentration of oxygen in the solution if you haven't already done so. Ask your lab instructor or course advisor for his preferred method. Examples of methods include the "Probe and Meter" method, or the "Azide-Winkler" method.
Convert the concentration of oxygen in the solution to PPM if you measure it in M, or moles per liter. A mole of molecular oxygen -- O2 -- is equal to 6.022 x 10^23 atoms of oxygen, or 32.00 grams of oxygen. If the solution is 0.2 M, in other words 0.2 moles of oxygen per liter of solution, do the calculation as follows:
0.2 mol O2 x 32.00 g per mol O2 = 6.40 g O2
6.40 g x 1,000 mg per gram = 6,400 mg O2 = 6,400 PPM O2.
Divide the total mass of oxygen in grams in the solution by the PPM concentration to get to the total volume of the concentration. If you know the solution contains 100 grams -- in other words, 100,000 mg -- of oxygen, do the calculation as follows:
100,000g O2/6,400 PPM O2 = 15.62 L solution.