When does Oracle index null column values?

I used to think that Oracle does not index a row when one of the column values is null.

Some simple experimentation shows this to be not the case. I was able to run some queries unexpectedly accessing only indexes even though some columns were nullable (which of course was a pleasant surprise).

A Google search led to some blogs with conflicting answers: I have read that a row gets indexed unless all indexed columns are null, and also that a row gets indexed unless the leading column value for the index is null.

So, in what cases does a row not enter an index? Is this Oracle version specific?