We use femtosecond mid-infrared pump-probe spectroscopy to investigate the effects of the osmolyte trimethylamine N-oxide (TMAO) on the structural dynamics of water. As a comparison, we also investigate the effects of other amphiphilic molecules: tetramethylurea (TMU), urea, proline, and N-methylacetamide (NMA). Our measurements show that TMAO has the unique property of increasing the orientational mobility of part of the water molecules in the solution, indicating that TMAO reorganizes the hydrogen-bond network of water in a special way. We also investigate the influence of the simultaneous presence in a solution of TMAO and urea, and of TMAO and NMA. It turns out that the effects of TMAO and urea are additive, whereas those of TMAO and NMA are nonadditive.