Nanomechanical oscillators are at the heart of a variety of precision measurements. This article reports on dispersive radiation coupling of nanomechanical oscillators to the evanescent near-field of toroid optical microresonators. The optomechanical coupling coefficient which reaches values > 200 MHz/nm, corresponding to a vacuum optomechanical coupling rate > 4 kHz, is characterized in detail and good agreement between experimental, analytical and finite element simulation based values is found. It is shown that both the mode-structure and -patterns of nanomechanical oscillators can be characterized relying solely on Brownian motion. Moreover, it is demonstrated that the radiation pressure interaction can cause self-sustained coherent nanomechanical oscillations at nano-Watt power levels as well as cooling of the nanomechanical oscillator. Finally, the feasibility of coupling nanomechanical motion to two optical modes where the optical mode spacing exactly equals the mechanical resonance frequency is demonstrated for the first time. As shown here, this Raman-type scheme allows both amplification and cooling. (C) 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.