Step 1 We identify the starting point in the equation.

This is always the point that we can calculate an amount of substance. In this case we know we have 5 moles of water.

Step 2 Identify the finish.
This is always the subject we need to find out. In this case it is the amount of oxygen in mole

Step 3 Using the coefficients in the balanced equation derive the ratio. Which is always finish divided by start.
1/2 = 0.5

Step 4 Multiply the ratio by the number of moles given at the start.

ratio X start = moles of oxygen required

1/2 X 5 moles of water = 2.5 moles of oxygen gas required