That depends on what the volume of the initial solution is.
In order to be able to determine how much water is needed to reduce your solution's osmolarity from 520 mOsmol/L to 300 mOsmol/L, you need the actual volume of the initial solution.
Osmolarity expresses the number of osmoles per liters of solution.
In your case, you need to go from a more concentrated solution to a diluted solution. Let's assume that you have
The number of osmoles this solution would contain would be equal to
Now, let's assume that
This is equivalent to
So, to get how much water you need to add, simply multiply the initial volume by 0.73.
For example, if you start with a 1-L solution, you need to add
of water to go from 520 mOsmol/L to 300 mOsmol/L.
This of course implies that the volume of the final solution will be