Let Ω be a bounded C 2 domain in R n and uin viscosity sense, where 0 ≤ a ≤ A0, C0, K > 0, and I is a suitable nonlocal operator. We show that u/δ is in C κ ( Ω) for some κ ∈ (0, 1), where δ(x) = dist(x, Ω c ). Using this result we also establish that u ∈ C 1,γ ( Ω). Finally, we apply these results to study an overdetermined problem for mixed local-nonlocal operators.