In this paper, we consider the oscillation theory for fractional differential equations. We obtain oscillation criteria for three classes of fractional differential equations of the forms T t 0 α x(t) + m i=1 p i (t)x(τ i (t)) = 0, t ≥ t 0 , T t 0 α (r(t)(T t 0 α (x(t) + p(t)x(τ (t)))) β) + q(t)x β (σ (t)) = 0, t ≥ t 0 , and T t 0 α (r 2 T t 0 α (r 1 (T t 0 α y) β))(t) + p(t)(T t 0 α y(t)) β + q(t)f (y(g(t))) = 0, t ≥ t 0 , where T α denotes the conformable differential operator of order α, 0 < α ≤ 1.