The orbit dimension $σ(G)$ (also called the separation number or rigidity index) of a permutation group $G$ with domain $Ω$ is the minimum cardinality of a subset $S \subseteq Ω$ such that, for any two distinct elements $ω,ω'\in Ω$, there exists $α\in S$ for which $ω$ and $ω'$ lie in distinct orbits of the stabilizer $G_α$. In this paper, we first observe that if $G$ is transitive, then $σ(G)\le |Ω|-r+1$, where $r$ is the rank of $G$, and we obtain strong structural information on the groups for which equality holds. Next, we investigate the orbit dimension in the case where $G$ is the symmetric group of degree $n$, acting on the set of $k$-subsets of $\{1,\ldots,n\}$. In this case, this invariant equals the metric dimension of Johnson graphs.