If you have not read or heard about CDB or PDB I recommend a few references below.
Add CDB to Enterprise Manager 12c
1. Login to OEM
2. Click setup>add target>add targets manually
7. Set the dbsnmp password and click test connection.
If you receive a message that the dbsnmp account is locked click the change dbsnmp password button and set a new password to unlock account.
10. Click next
11. Click save
You can see this same demo in my YouTube video below.
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References:Oracle Database 12c Multitenant Architecture Overview
Install Oracle 12c (12.1) + New Features and Tips
Oracle 12c Documentation Library