"OR" operator ranking


Hi All,
the ranking criteria for “OR” operator seems to be counterintuitive.
If I seach:

foo bar

I get results with both “foo” and “bar”. Nice.

If I search:

foo OR bar

I get results with either “foo” or “bar”, nice. BUT I would expect to get first results with both “foo” AND “bar”, they should have higher ranking. This is a normal ranking strategy for search engine.

Can I set some parameter to obtain such behavior?

Thanks a lot