Revised Question: How do I prevent Oracle 12c Database from starting on machine boot
You'll want to prevent the OracleService&lt;&lt;SID&gt;&gt; from starting automatically.  Assuming you used the default SID of ORCL, that would be OracleServiceORCL.

You'll likely also see a number of other services that start with Oracle that are for various other components (i.e. your listener, your MTS recovery service, etc.)  You can set all those to not start automatically if you want.  Given that those services tend to use many fewer resources, it's generally easiest to let them run to make it easier to start the database later on.