Startup oracle database 12c manually with SQL*Plus command not when OS loads
You can configure this behaviour through the Administration Assistant, where you can choose to start (or not to start) the database instance with the Windows service. 

Alternatively, you can do this with oradim, e.g:

oradim -edit -sid ORCL -startmode manual


The Windows service should be set to start automatically, and with the above setting, the database instance will not start with it.