The name of the company that performs the drilling or the responsible party.
In many cases, the most recently used name of the operator will not have been entered in the PDMS, if the name change was not associated with a new well activity.
The best source for current operator names is the Division of Oil and Gas (DOG). See their Active Operator List at < http://www.in.gov/dnr/dnroil/files/og-ActiveOperatorList.pdf >