We can develop reports on OLTP...but it may lead to performance issue because the no.of joins are more.
Take the OLTP as DB -->Import the tables in the RPD and build the data model and do changes as required.
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I'd follow this whole series of blogs: http://www.rittmanmead.com/2012/12/obiee-transactional4/
He goes through a lot of details as far as reporting on transactional tables go in his 4 blog posts so far.
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