According to Graham's Law of Diffusion,"the rates of diffusion of two gases are inversely proportional to the square root of their Molar masses or Densities at the same pressure and temperature".
                                     r₁ / r₂  =  [tex] \sqrt{M2 / M1} [/tex]
Where,
            r₁  =  Rate of Helium
            r₂  =  Rate of Oxygen
            M₂  =  Molar mass of Oxygen  =  32 g/mol
            M₁  =  Molar mass of Fluorine  =  4 g/mol
Putting values,
                                           r₁ / r₂  =  [tex] \sqrt{32 / 4} [/tex]
                                           r₁ / r₂  =  [tex] \sqrt{8} [/tex]
                                           r₁ / r₂  =  2.82
Result:
            Helium gas effuses 2.82 times faster than Oxygen gas.