In this paper we present an implicit dictionary with the working set property i.e. a dictionary supporting insert(e), delete(x) and predecessor(x) in ${\mathcal O}(\log n)$ time and search(x) in ${\mathcal O}(\log\ell)$ time, where n is the number of elements stored in the dictionary and ℓ is the number of distinct elements searched for since the element with key x was last searched for. The dictionary stores the elements in an array of size n using no additional space. In the cache-oblivious model the operations insert(e), delete(x) and predecessor(x) cause ${\mathcal O}(\log_B n)$ cache-misses and search(x) causes ${\mathcal O}(\log_B \ell)$ cache-misses.