By default, adding filters to the query object will use the AND operator. To use OR, you will need to add a SolrFilterSubQuery.

 * Implements hook_apachesolr_query_prepare().
function mymodule_apachesolr_query_prepare($query) {
  // filter search to show Article OR Page content types
  // Create sub query to handle "OR"
  $sub_q = new SolrFilterSubQuery();
  // OR operator is used by default
  $sub_q->addFilter('bundle', 'article');
  $sub_q->addFilter('bundle', 'page');
  // apply the query