This article as one of the first in legal – not only polish-language – literature presents a comparative analysis of the issue of a head of state’s oath of office. The paper underlines differences, similarities and peculiarities in legal regulations of the institution of the oath of office in regard to European presidents and monarchs. The study touches especially on such topics as the oath formula, its optional or obligatory religious dimension, the time of taking the oath, the subject which officially receives it, consequences of taking the oath and legal effects of the oath’s infringement. The basic normative analysis, both on constitutional and statutory level, was supplemented with remarks on the practice of the given institution, for example the oath’s taking ceremony. Description of presidential or royal authority in particular states is not complete without a reference to the institution of the oath of office. The arguments undertaken in the article aim to prove that the title subject cumulates several relevant issues, which are noteworthy for jurisprudence. Taking the oath of office by any president or monarch is not so much a solemn symbolic event as conventional activity important in view of the legal order.