Cumulative totalling

I know what formula to use when working out the sum of a range but when I copy the formula down it shows the cumulative total of the populated range in all the cells. Now I want a condition that says when the adjacent cell is blank then show nothing. I'm sure this must be easy and I probably should know how to do this but my searches have yielded no results. Please look at my attachment.

Basically to summarise: Cell P6 should show nothing until a value appears in O6 at which point it should show the cumulative total of range O3:O6

I hope this makes sense!

Thanks in advance