There are two major things to care when clustering Odoo, sessions and attachments. For sessions you need to have an external shared session storage, like Redis. For the attachments you need to store them into a shared disk, you could also store your sessions into a shared disk too. In my experience the session storage like Redis need an Odoo Session Store that need to be set to use into the Odoo code itself when your Odoo is configured with workers. But to answer the question, yes, Odoo can be clustered you just need to access the Odoo with a reverse proxy load balancer like nginx that allow you to config more than one backend to access to domain or ip address of the vhost