I have implemented CI on SharePoint server projects without a problem (so to speak). Can this also be done for SharePoint online?
If it is a large project then of course if you are making a solution.
This of course totally depends on how you are doing your ci.
If you are doing it using your source control only with separate development servers (This is how we do it) and have the lead developer push to 365/staging.
Detail your ci methodology and work practice and I can elaborate further. But basically we use seperate dev > regular integration > lead staging (This can be 365).
CI is purely to reduce the risk of conflicts when you have multiple developers, usually it's always the .proj file if nothing else.