Oops, just noticed the date, never mind.

How many rows are returned without the WHERE clause?

I may be wrong, but I think the openquery will filter the results in Oracle, whereas the query in your post wants to move all the rows over to sql, then filter on the WHERE clause.

Maybe there is a record containing incompatible data that is filtered out by the where clause. The openquery doesn't attempt to return it, but the normal query is trying to bring it across. Can you select all the data from both tables individually using the linked server?