List of OS available for each VPS model https://www.ovh.co.uk/vps/operating-system.xml#os-list
You can scale the VPS accordingly to your requirements from VPS classic 1 to 4 and VPS cloud 1 to 4 in the 2014 range. It is not possible to cross over from one to another such as going from classic to cloud. You would need to complete an new order if you wish to move from classic to cloud. This would involve a process of manual migration of data to the new VPS server.