Generally in Drupal query builder, we can chain condition() method to add AND conditions to the query we are building.
Which will create a query something like 
WHERE Condition1 AND Condition2 AND Condition3	
by following code:
condition(Condition1)->condition(Condition2)->condition(Condition3); ?>
 
Now If we want OR,
condition(db_or()->condition(Condition1)->condition(Condition2)->condition(Condition3)); ?>
 
If we want to nest OR inside AND,
something like 
WHERE (Condition1 OR Condition2) AND (Condition3 OR Condition4)	

You can do:
condition(db_or()->condition(Condition1)->condition(Condition2));
$query->condition(db_or()->condition(Condition3)->condition(Condition4));
?>
 
If we want to nest AND inside OR,
something like 
WHERE (Condition1 AND Condition2) OR (Condition3 AND Condition4)	
You can do:
condition(db_or()->condition(db_and()->condition(Condition1)->condition(Condition2))
      ->condition(db_and()->condition(Condition3)->condition(Condition4)));
?>