Consider a distribution of pebbles on a connected graph $G$. A pebbling move removes two pebbles from a vertex and places one to an adjacent vertex. A vertex is reachable under a pebbling distribution if it has a pebble after the application of a sequence of pebbling moves. The optimal pebbling number $π^*(G)$ is the smallest number of pebbles which we can distribute in such a way that each vertex is reachable. It was known that the optimal pebbling number of any connected graph is at most $\frac{4n}{δ+1}$, where $δ$ is the minimum degree of the graph. We strengthen this bound by showing that equality cannot be attained and that the bound is sharp. If $\operatorname{diam}(G)\geq 3$ then we further improve the bound to $π^*(G)\leq\frac{3.75n}{δ+1}$. On the other hand, we show that for arbitrary large diameter and any $ε>0$ there are infinitely many graphs whose optimal pebbling number is bigger than $\left(\frac{8}{3}-ε\right)\frac{n}{(δ+1)}$.