Partial pressure is found by Dalton's Law:

Px = PTotal ( nx / nTotal )

where

Px = partial pressure of gas x

PTotal = total pressure of all gases

nx = number of moles of gas x

nTotal = number of moles of all gases

Step 1

Find PTotal

Standard pressure is 1 atm.

Step 2

Add up the number of moles of the component gases to find nTotal

nTotal = noxygen + nnitrogen

nTotal = 100 + 900

nTotal =1000cm3

Step 3

Now you have all the information needed to plug the values into the equation and solve for Poxygen

P-oxygen = PTotal ( n-oxygen / nTotal )

oxygen = 1 atm ( 100 / 1000 )

oxygen= 0.1atm