You have two options here. The first option is to use auditing on the tables in question. For instance, to audit UPDATES on the SCOTT.EMP table, issue the following command:
AUDIT update ON scott.emp WHENEVER SUCCESSFUL;Whenever anyone performs a successful UPDATE command on SCOTT.EMP, an entry will be logged in the DBA_AUDIT_TRAIL view. There are many options with the AUDIT command, so please consult the Oracle documentation. This option is more preventative than reactionary. You have to turn on auditing before you can see the results.
The second option is to use LogMiner to see the operations. This option is more reactionary as it can be performed after the fact. LogMiner is quite a lengthy topic, so I will refer you to the relevant section in the Oracle Utilities Guide.