Parameters given:

Number of moles of Cyclopropane, N\(_c\) = 0.75 mole

Number of moles of Oxygen, N\(_o\) = 0.66 mole

Total pressure of mixture of gases, P\(_T\) = 0.70 atm

Partial pressure of Oxygen in the mixture, P\(_O\) = ?

Using the formula, P\(_O\) = \(\frac{N_o}{{N_o} + {N_c}}\) X P\(_T\)

 = \(\frac{0.66}{{0.66} + {0.75}}\) X 0.70

 = \(\frac{0.66}{1.41}\) X 0.70

 = 0.3276 

 = 0.33 atm