def METHOD_NAME(self, variables):
    param = self.param
    eps_s = variables["Separator porosity"]
    eps_p = variables["Positive electrode porosity"]
    eps = pybamm.concatenation(eps_s, eps_p)
    deps_dt_s = variables["Separator porosity change [s-1]"]
    deps_dt_p = variables["Positive electrode porosity change [s-1]"]
    deps_dt = pybamm.concatenation(deps_dt_s, deps_dt_p)
    c_ox = variables[
        "Separator and positive electrode oxygen concentration [mol.m-3]"
    ]
    N_ox = variables["Oxygen flux [mol.m-2.s-1]"].orphans[1]
    a_j_ox = variables[
        "Positive electrode oxygen volumetric interfacial current density [A.m-3]"
    ]
    source_terms = pybamm.concatenation(
        pybamm.FullBroadcast(0, "separator", "current collector"),
        param.s_ox_Ox * a_j_ox,
    )
    self.rhs = {
        c_ox: (1 / eps)
        * (-pybamm.div(N_ox) + source_terms / param.F - c_ox * deps_dt)
    }