In Bomford v. Socony Mobil Oil Co., 1968 OK 43, 440 P.2d 713, the Oklahoma Supreme Court defined "due diligence" this way:
"Due diligence is a relative term lacking a fixed content. It presents a question for judicial determination which must be decided in the first instance by the trial court. Its jurisdiction over the absent defendants clearly depends upon the resolution of that issue. And in order to decide the matter the trial judge must have the facts before him. If the facts before the court have a legal tendency to show a diligent search for the whereabouts of the defendant, or the whereabouts and identity of his heirs, and the court is satisfied that primary sources at hand, such as local tax rolls, deed records, judicial records and other official records, as well as available secondary sources, such as a telephone directory, a city directory or the like, have been exhausted in a meaningful pursuit of information, the approval of publication process as a method of notification is to be regarded with the same effect as a decision upon any other fact issue submitted for judicial determination. But an inquiry and determination there must always be." Id.