A tablespace can be Audited in 12c by creating the audit policy for the tablespace action.

1. Create audit policy:

SQL> create audit policy pol2 actions create tablespace, drop tablespace, alter tablespace;

2. Enable audit policy for SYSTEM user:

SQL> audit policy pol2 by SYSTEM;

3. Connect to the database and run few alter statements.

SQL> conn system/[password]
Connected.
SQL> CREATE TABLESPACE tbs1 DATAFILE 'tbs1_data.dbf' SIZE 1m;
Tablespace created.

SQL> ALTER TABLESPACE tbs1 OFFLINE NORMAL;
Tablespace altered.

SQL> ALTER TABLESPACE tbs1 ONLINE;
Tablespace altered.

SQL> ALTER TABLESPACE tbs1 BEGIN BACKUP;

4. The results should be showing up in the UNIFIED_AUDIT_TRAIL view.

SQL> SELECT DBUSERNAME, SQL_TEXT, EVENT_TIMESTAMP FROM UNIFIED_AUDIT_TRAIL where DBUSERNAME like 'SYSTEM';
DBUSERNAME   SQL_TEXT                                                  EVENT_TIMESTAMP
------------ --------------------------------------------------------  -----------------------------
SYSTEM       CREATE TABLESPACE tbs1 DATAFILE 'tbs1_data.dbf' SIZE 1m   30-JAN-20 08.06.20.601459 PM
SYSTEM       ALTER TABLESPACE tbs1 OFFLINE NORMAL                      30-JAN-20 08.07.04.068434 PM
SYSTEM       ALTER TABLESPACE tbs1 ONLINE                              30-JAN-20 08.07.16.330197 PM
SYSTEM       ALTER TABLESPACE tbs1 BEGIN BACKUP                        30-JAN-20 08.10.42.288961 PM