Can we update multiple record with join of two query
Try merge instead, e.g.

merge into oiu o
  using (select u.usr_key
         from user u
         where u.usr_status = 'Active'
           and u.usr_udf_isexecutiveuser = '1'
        ) x
  on (o.usr_key = x.usr_key)
  when matched then update set
    o.oiu_prov_mechanism = 'RECONCILIATION',
    o.pol.key = null,
    o.oiu_policy_based = null,
    o.oiu_policy_revoke = null
  where o.oiu_prov_mechanism = 'AP HARVESTED';