Cloud data centers operate at very low utilization rates resulting in significant energy waste. Oasis is a new approach for energy-oriented cluster management that enables dense server consolidation. Oasis achieves high consolidation ratios by combining traditional full VM migration with partial VM migration. Partial VM migration is used to densely consolidate the working sets of idle VMs by migrating ondemand only the pages that are accessed by the idle VMs to a consolidation host. Full VM migration is used to dynamically adapt the placement of VMs so that hosts are free from active VMs. Oasis sizes the cluster and saves energy by placing hosts without active VMs into sleep mode. It uses a low-power memory server design to allow the sleeping hosts to continue to service memory requests. In a simulated VDI server farm, our prototype saves energy by up to 28% on weekdays and 43% on weekends with minimal impact on the user productivity.