The document to configure the custom login page OAM is -> "How To Configure A Custom Login Page For OAM 11g (Doc ID 1281026.1)" -- This is for 11g, but a similar method may be used for 12C as well..
IF you wan to motify the standard login page directly, then you will have to find it on the filesystem and do your modifications accordingly. I didn't test it yet, but that page is probaby in oam-server.ear.. Check this blog post to get an idea about it ->
https://sudhirtadi.wordpress.com/2014/11/13/oracle-access-manager-password-management-2