Single-crystalline cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) nanoneedles and nanorods were prepared by a solution-phase approach in the presence of an ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4). [BMIM]BF4 can act as a cosolvent, structure-directing agent, and reducing reagent in the reaction system. Based on the redox reaction of MnCl2 and KMnO4 in the mixed solvents of water and [BMIM]BF4, the formation of OMS-2 nanoneedles followed the rolling mechanism with lamellae as an intermediate. However. the direct reaction of KMnO4 with [BMIM]BF4 resulted in the formation of OMS-2 nanorods with diameters as small as 3-6 nm. The formation mechanism of OMS-2 nanostructures was discussed.