The oaths are one of the occasions that people need throughout their lives in order to strengthen their words and reinforce their claims. The oath, which has been used since the beginning of humanity for purposes such as confirming the promise and determination and convincing the interlocutor, is accepted as legitimate in Islam. However, it has been regulated by centring on the belief in tawhid and it has been stated that oaths contrary to this belief are invalid. Throughout human history, there have been many forms of oaths and expression patterns. The expression patterns used as oaths have differed according to societies. Today, this difference is also noticeable and it is seen that the wording of oaths varies from region to region. In addition to this, the fact that some fiqh sources emphasise that oaths are based on custom points to the relationship between oath and custom. This necessitates bringing the effect of custom on the judgement of oaths to the agenda. As it is known, in matters based on custom, custom has an important effect on the determination of jurisprudential judgement. Considering that the wording of oaths varies according to cultures and geographies, it is understood that custom is also effective here. However, the fact that the effect of custom on religious judgements is limited to not contradicting the basic principles of religion and the relevant evidence makes it necessary to examine the issue meticulously. In order to analyse the effect of custom on oaths, the words of oaths must first be classified. Because addressing the relationship between oath and custom without this classification will cause disorganisation and make it difficult to reach clear conclusions, as it will require examining each phrase separately. When we look at the fiqh sources, it is seen that the words of oath are dealt with in a disorganised manner and the evaluations on the subject are interspersed between the lines. This is where the difficulty of making a judgement on the issue comes from. In this study, in addition to the types of oaths, the relationship between oaths and custom in the context of expressing judgement is discussed. The first aim of the study is to classify the words of oath by considering certain criteria. Another aim is to determine whether custom is effective in the production of judgement through the expression patterns that emerge as a result of this classification. In our study, oaths are discussed in two parts in terms of requiring expiation and the expression patterns used. Since there is sufficient information in the sources about the types of oaths in terms of requiring expiation, this subject has been summarised. After that, the words of oath, which are scattered in the sources, are categorised by taking into account the common points between them. In this framework, the words of oaths are divided into three main parts: oaths made in the name of Allah, oaths made on beings other than Allah, and oaths in the form of conditional sentences. While the oaths made in the name of Allah are classified as oaths made with the names and attributes of Allah, the oaths made on other than Allah are classified as oaths made on Ka'ba, prophets, and the Qur'an. The words of oaths in the form of conditional sentences are analysed under three headings as the words expressing forbidding the lawful, apostasy and cursing. Under the evaluation sections following each main heading, the evidence on the subject has been analysed and certain determinations have been made. In summary, it has been concluded that custom is effective both in the meaning of the sentences used as oaths and in the validity of the oaths made with the attributes of Allah. It is concluded that the oath made on the Qur'an is also considered within the scope of the oath made on the attributes of Allah and will be valid if custom is formed. The oaths made on the Ka'bah and the prophets, on the other hand, were accepted invalid, taking into account the scriptures stating that it is not permissible to swear on anyone other than Allah. Among the words of oaths in the form of conditional sentences, the words of making the lawful unlawful and apostasy are considered as oaths, whereas the phrases containing curses are not considered as oaths, whether custom is formed or not. However, the justification for this has been the evaluations related to the nature of the oath rather than custom. [ABSTRACT FROM AUTHOR]