This method of collecting oxygen is called water displacement method.When you collect oxygen from this method in the container where oxygen is collected the total pressure is atmospheric pressure.Not only oxygen but also water vapour will contribute to this pressure. So finally this atmospheric pressure consist of pressure of oxygen and pressure of water vapour.
To get the vapour pressure at various temperatures we have vapour pressure charts for water. It is referenced here.
In this case water vapour pressure at 18C is 2.06kpa.
Total pressure = pressure of oxygen+pressure of vapour
`101.3 = 2.06+P_(O_2)`
`P_(O_2) = 99.24kpa`
So the pressure of oxygen is 99.24kpa.