You can use this For making OR between 2 conditions ByDefault it is AND
$query=db_select('users','u')->fields('u',array('uid','title','created','uid'));
$query->join('flag_content','fc' , 'u.uid = fc.content_id');
$db_or = db_or();
$db_or->condition('fc.fid', '5' , '=');
$db_or->condition('fc.uid', $uid , '=');
$query->condition($db_or);
$result = $query->execute()->fetchAll();
It is possible to have AND condition within OR suppose you have 3 conditions and if you want it to be like con1 AND con2 OR con3 and even many other cases in this case you can use db_and like db_or because when you are trying to use db_or all the conditions within db_or takes now in case you want and between those you can use db_and that is little tedious but its possible to implement it. Here is the example below
$query = db_select('work_details_table','wd');
$query->join('work_table','w','wd.request_id = w.request_id');
$query->fields('w',array('client_id'));
$query->fields('wd');
$query->condition('wd.request_id',$request_id);
if($status == 5 || $status == 6 || $status == 7){
$query->condition('wd.status',$status);
}elseif($user_type == 'writer'){
$query->condition('wd.writer_id',$user_id);
$db_or = db_or();
$db_and = db_and();
$db_and->condition('wd.status',2);
$db_and->isNull('wd.editor_id');
$db_or->condition('wd.status',4);
$db_or->condition('wd.status',1);
$db_or->condition($db_and);
$query->condition($db_or);
}else if($user_type == 'editor'){
$query->condition('wd.editor_id',$user_id);
$db_or = db_or();
$db_and = db_and();
$db_and->condition('wd.status',2);
$db_and->isNotNull('wd.editor_id');
$db_or->condition('wd.status',3);
$db_or->condition($db_and);
$query->condition($db_or);
}
//-----------------------------------------------------------------------------------------
$completed_sku = $query->execute()->fetchAll();
This will give you an idea how the complex and or condition can be handled in Drupal PDO queries .