we are going to take a metro from Odeon to Les Halles and then RER A from Chatelet - Les Halles to Disnyland ( Gare de Marne La Vallee Chessy). we should buy separate ticket to the Metro and to the RER A?
You can purchase a single RER ticket for the entire journey on both Paris Metro and Paris RER, using this ticket for transfers (passing through turnstiles and gates) between the two train lines. As long as you don't exit any station's gates to the outside/surface you can continue to use that same ticket from Odeon, all the way through to Disneyland / Marne La Vallee station.
It's easiest to buy this RER / Origin-Destination ticket from the ticket agent in a ticket booth at the Odeon Metro station rather than trying to use a train ticket machine (navigating through the screens for this type of ticket may not be easy).