Comparison of the local OPR profiles in the PDR in models with P = 10⁷ K cm^-3 and χ = 10³ for the four prescriptions for ortho-para conversion on grains: no conversion (purple), rate equation approach neglecting the fluctuations (green), statistical approach taking the fluctuations into account (blue), and full conversion for H₂ molecules sticking to grains (red). The temperature profile of the PDR is also shown for reference (dotted black line), along with the corresponding LTE OPR profile (solid black line).