Performance tuning of a system

I'm facing with a nice (at least for me) exercise of performance and tuning. I have an OLTP system must process each transaction in no more than 2 seconds. A transaction is not a single query but consists of some query and store procedure run from an application server. Almost 98% of this complete in less than 2 second. The main concern is that the system is accessed also by third parties application, mainly for enquiries but also for updates. The DBA has never been involved in the evaluation of DML done by third parties applications and there is

Now I would like, as starting point, to trace for 24hrs all queries done by user THIRDPARTYAPP1 and THIRDPARTYAPP2 and its duration in order to understand if some query run by these application might in some case impact the performance of the database; also this may offer an opportunity of review cause there is no complete list of which queries are done by other external applications. Moreover any other hints is well accepted.

Database is 11.2.0.2 under Linux.

Thanks
Ste

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