Recently Kubica et al. (Inf. Process. Let., 2013) and Kim et al. (submitted to Theor. Comp. Sci.) introduced order-preserving pattern matching: for a given text the goal is to find its factors having the same 'shape' as a given pattern. Known results include a linear-time algorithm for this problem (in case of polynomially-bounded alphabet) and a generalization to multiple patterns. We give an O (n log log n) time construction of an index that enables order-preserving pattern matching queries in time proportional to pattern length. The main component is a data structure being an incomplete suffix tree in the order-preserving setting. The tree can miss single letters related to branching at internal nodes. Such incompleteness results from the weakness of our so called weak character oracle. However, due to its weakness, such oracle can answer queries on-line in O (log log n) time using a sliding-window approach. For most of the applications such incomplete suffix-trees provide the same functional power as the complete ones. We also give an O (n log n/log log n) time algorithm constructing complete order-preserving suffix trees.