Actually it seems that CMIS was half-implemented in Office365, but Microsoft voluntarily disabled it (URL broken after Microsoft reorganized their forums).
CMIS is a protocol for interoperability, and it seems like Microsoft wants to prevent interoperability for Office365, presumably to prevent locked-in customers from migrating to more open solutions like Alfresco Cloud.
Companies who want CMIS interoperability should stay away from Office365 and install SharePoint on-premise.
Please alsoI asked Microsoft to implement CMIS in all SharePoint products, and it became the ask Microsoft to implement CMIS in all SharePoint productshighest-voted idea on UserVoice. Facing that, Microsoft deleted it, showing that they still consider vendor lock-in as a feature rather than a bug. Let's not loose hope that Microsoft will one day become a bit more open, so please vote for the re-created idea.