If this a combined and concurrent responsibility then the person(s) must absoultely be a member of Top Management per the most stringent requirement of the 3 standards cited, OHSAS 18001....No brainer, no guesswork, no debate.
However, nothing prevents delegation of responsibility for day-to-day operation to a lower level person, but ultimate accountability & responsibility (2 entirely seperate things) must be with Top Management
I see it all the time when I do integrated audits of Q/E/OHS systems, and I have issued nonconformities that have stood the test when challenged if TM is not designated the MR of a Q/E/OHS integrated system