How do you monitor specific SQL statement of a specific user ? We have a few standalone users here which cause high server loads for several 11.1.0.7 Oracle's. 
I spoke with them a few times but it seems that their queries are causing high loads and probably bottlenecks

Is there any option in Oracle to monitor or see which SQLs they running ? I'm not talking about only the ones which cause high loads, although they're the most important ones I would like to see all of their queries.
You might want to look into AUDIT ALL STATEMENTS BY user. As per Oracle's documentation, audit all statements by ...  audits all top-level SQL statements executed. Top-level SQL statements are issued directly by a user. SQL statements run from within a PL/SQL procedure or function are not considered top-level statements. Therefore, this clause does not audit the statements executed within PL/SQL procedures or functions. However, the execution of the PL/SQL procedure or function itself is audited. This clause is useful if you want to audit all the statements in a specific environment, regardless of other auditing configurations that are system wide or user specific.