Stackoverflow: Does oracle allow the uncommitted read option?
Tom provides a great answer to this: On Transaction Isolation Levels
The READ UNCOMMITTED isolation level allows dirty reads. Oracle Database doesn't use dirty reads, nor does it even allow them. The basic goal of a READ UNCOMMITTED isolation level is to provide a standards-based definition that allows for nonblocking reads.
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Now, a database that allowed a dirty read ... not only does it return the wrong answer, but also it returns ... [an answer] ... that never existed in the table. In a multiuser database, a dirty read can be a dangerous feature. Personally, I've never seen the usefulness of it... The point here is that dirty read is not a feature; rather, it's a liability. In Oracle Database, it's just not needed. You get all of the advantages of a dirty read—no blocking—without any of the incorrect results.
But unfortunately, Oracle does NOT implement Serializability correctly, it implements Snapshot Isolation instead. PostgreSQL does!
Probably, this will never change! :-(