How can I do an OR operation on the values of a solr field? Say I have a facet on countries, I'd like to be able to click on more than one country (say US and UK) and get results tagged "US" OR "UK".

Currently If I click on "UK" on the facet block, the other terms disappear if this was a "single solr field".

And if it was a multi-valued solr field, the other terms don't disappear but an AND operation is done.

How can I change the default behavior to allow OR operation on both "single" and "multi-valued" solr fields?

I'm using the Apache Solr Search Integration module version 1.6.
Can't say for sure if this is in 1.6 but if you configure the country facet block you'll get the option to use AND or OR inside the facet.