<p>I have to simultaneously load data into a table and run queries on it. Because of data nature, I can trade integrity for performance. How can I minimize the overhead of transactions?</p>

<p>Unfortunately, alternatives like MySQL cannot be used (due to non-technical reasons). </p>

You want transaction isolation read uncommitted. I don't recommend it but that's what you asked for :)
This will allow you to breach transaction isolation and read uncommitted inserted data.
Please read this Ask Tom article:
http://www.oracle.com/technology/oramag/oracle/05-nov/o65asktom.html
.
UPDATE: I was actually mistaking, Oracle doesn't really support read uncommitted isolation level, they just mention it :).