There are different strengths of CBD oil. Generally, CBD oil is sold in bottles containing a certain number of milligrams of CBD, so you may see 750mg, 1500mg, 3000mg bottles and so on. This number references the total milligrams of CBD in the bottle, not per dose.
Measuring strength is dependent upon the size of the bottle. For example, a 2oz. bottle that contains 1500mg of CBD is half as concentrated as a 2oz. bottle that contains 3000mg of CBD. This means a 1mL dose of the 1500mg bottle is half as “strong” as 1mL from the 3000mg bottle.