In this paper we prove that for non-negative measurable functions f, \(\begin{aligned} I_\alpha f \in BMO({\mathbb {R}}^n) \text { if and only if } I_\alpha f \in BMO^\beta ({\mathbb {R}}^n) \text { for } \beta \in (n-\alpha ,n]. \end{aligned}\) Here \(I_\alpha \) denotes the Riesz potential of order \(\alpha \) and \(BMO^\beta \) represents the space of functions of bounded \(\beta \) -dimensional mean oscillation introduced in [12].