Okla. Stat. tit. 38 § 21
If a grand jury is ordered, the number stated in the judge's order, not to exceed one hundred, shall be summoned as grand jurors, and the grand jury shall be impaneled from said persons. The judge of the district court shall order the court clerk, one of his or her deputies or the court administrator to order the stated number of jurors from the Administrative Director of the Courts. In any district court where an electronic jury management system is implemented pursuant to Section 13 of this act, the judge may order the stated number of jurors to be randomly selected and summoned by the court clerk or trial court administrator utilizing the jury management system, and the district court may use the jury management system to randomly select the names of the grand or petit jurors and alternate jurors from the panels so drawn. In addition to the twelve (12) grand jurors to be impaneled, three additional persons shall be selected as alternate grand jurors. The alternate grand jurors shall attend all functions of the grand jury during its term and shall be subject to all laws governing grand jurors. Provided that, no alternate juror shall participate in any deliberations of the grand jury until appointed to fill a vacancy. If the judge so directs, the persons summoned for the grand jury panel who are not used thereon may be transferred to the petit jury panel. Additional and other drawing of as many names as the court may order may be had at any such time as the court or judge may order for the completion of a grand or petit jury panel, or for the impaneling of a new grand or petit jury if, in the judgment of the court, the same shall be necessary, or if, for any cause, the court, in its discretion, shall deem other jurors necessary. The court may excuse or discharge any person drawn and summoned as a grand or petit juror whenever, in its discretion, such action shall be deemed expedient. No person may be required to render service as a petit juror for more than one (1) day in any one calendar year unless he or she is selected to serve in a trial or is under consideration to serve in a trial and such consideration covers a period of two (2) or more days. Once selected, a juror shall serve on the jury for the duration of the trial unless excused by the presiding judge.
Okla. Stat. tit. 38, § 21