D4 & Oracle & sql parameters

I'm trying to perform a query against an Oracle8 db using Delphi 4.
This works:
select * from tablename where field='value'

When I do this:
select * from tablename where field= :var
set var to 'value' and activate the query I get the error message: Operation
not applicable.

I've created queries to Paradox, Access, and SQL Server without problems.
What am I doing wrong with Oracle??