Although the OCCP has Open in it's name it's not open at all, you cannot download the specification without becoming a member of the alliance (although that's not clearly stated but the download is password protected).
From what I found out that protocol probably is not meant to be implemented in microcontrollers as the Mega2560 but in full-fledged computers of the category of a Rapsberry Pi and up. For Linux there seems to exist an implementation.
Does Arduino have chipset on which OCPP is already ported?
That doesn't depend on the chipset.
It also needs a OS ported?
An Arduino doesn't have the resources to run an OS, at least not what OS means today.