How to specify AND, OR with dynamic query
I don't have the same structure of your table  but i test it with the following statement it works:

SELECT *
 FROM `node__body` AS n_b
 WHERE n_b.`deleted` = 0
 AND (
 n_b.`revision_id` IN (32,6,17)
 OR  n_b.`langcode` IN ('fr')
 );


here is the code snippet for your case:

  $query = $connection-&gt;select('inventory', 'inv')
    -&gt;fields('inv', ['part_number'])
    -&gt;condition('inv.visable', '1', '=');
  //- Set Or Query group
  $group = $query
    -&gt;orConditionGroup()
    -&gt;condition('inv.part_number', [SUBQUERY_1], 'IN')
    -&gt;condition('inv.part_number', [SUBQUERY_2], 'IN');
  //- Add Or group 
  $query-&gt;condition($group);
  $result = $query-&gt;execute()-&gt;fetchAll(PDO::FETCH_ASSOC);