There are many different ways to answer this question, but in general, O 5l in mL is referring to the volume of a liquid. The most common way to measure liquids is by using the metric system, which uses milliliters (mL) as its unit of measurement. In this case, O 5l in mL would be equivalent to 50 liters (L).
Another way to look at this question is to think about it from the perspective of converting between units. For example, 1 liter (L) = 1,000 milliliters (mL), so 50 liters (L) = 50,000 milliliters (mL). Thus, O 5l in mL is equal to 50,000 mL.