<p>In db2 a query with a 'with ur' clause allows the query to be an uncommitted read and so does the 'with nolock' clause in mysql. Is there such an option in oracle too... If not why??</p>

Tom provides a great answer to this:
On Transaction Isolation Levels
He says:
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.
...
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.