Index not used with or condition

Hi all,
    I am facing a strange problem when I run EXPLAIN against a table
having more than 100000 records. The query have lot of OR conditions
and when parts of the query is removed it is using index. To analyse
it I created a table with a single column, inserted 100000
records(random number) in it created index and run a query which
returns 1 record which have no or condition and it was using index.  I
added an OR conditon and is using sequential scan. I set the
enable_seqscan to off. I ran the tests again and is using index scan.
   So which one I have to use. Is this any bug in Explain.

rgds
Antony Paul.