We describe an experiment achieving radiation pressure excitation and cooling of a mechanical mode in a cryogenic Fabry-Perot cavity with a micro-mechanical oscillator (MEMS) as end mirror. The response function to periodic modulations of the intracavity power provides an independent measurement of the effective modal mass allowing an accurate estimate of the mode temperature from the corresponding displacement noise spectrum. We also obtained optical cooling of the MEMS fundamental mode at 110 kHz from 11 K to 4. 4 K, limited only by the optical Finesse and the mechanical quality of the system. These results represent a step towards the observation of quantum opto-mechanical effects and motivate further experiments with improved performances of the MEMS samples.