This article examines the evolution of the oath of office over an extended period, beginning in the XVI century and continuing to the present day. The author traces the origins of this legal phenomenon, suggesting that it initially emerged as a vow, applied more in private legal and procedural relations than in public-legal contexts. Since the oath was traditionally accompanied by the kissing of the cross, it was referred to as the “kissing of the cross” oath. During the period of the estate-representative monarchy, the oath of allegiance to the monarch held significant importance in the public sphere. Along with the crowning of the kingdom, it played a crucial role in legitimizing and strengthening public power. During the imperial period, the oath of office became widely practiced, with the military oath being the most typical example analyzed. This example demonstrates both continuity and innovation in legal practice. In Tsarist Russia, the oath was characterized by its estate-based nature, as it served as an instrument of estate policy and was administered separately to officers of noble birth and the rank and file, who were recruited from commoners. In the early Soviet period, the military oath was initially taken only by individuals from the working class, and its name was changed to the “solemn promise”. After 1939, the military oath was administered to all servicemen. The military oath played a vital role in establishing discipline, strengthening the morale, and enhancing the combat readiness of the army, thereby contributing to the consolidation of state power. At present, the military oath is not a thing of the past with the liquidation of Soviet power, but it requires improvement. Based on the analysis, the author attempts to formulate proposals for improving not only the military oath but also the civil service oath, as well as the procedure for its implementation.