The Oran fatwa was a responsum fatwa, or an Islamic legal opinion, issued in 1502 to address the crisis that occurred when Muslims in the Crown of Castile, in present-day Spain, were forced to convert to Christianity in 1500–1502. It was authored by a maliki mufti Ahmad ibn Abi Jum'ah, an Algerian scholar of Islamic law of the Maliki school; the term "Oran fatwa" was applied by modern scholars, due to the word "Al-Wahrani" ("of Oran") that appears in the text as part of the author's name.

The fatwa sets out detailed relaxations of sharia (Islamic law) requirements, allowing Muslims to conform outwardly to Christianity and perform acts that are ordinarily forbidden in Islamic law, when necessary to survive. It includes pliable instructions for fulfilling the ritual prayers, the ritual charity, and the ritual ablution, and recommendations when obliged to violate Islamic law, such as worshipping as Christians, committing blasphemy, and consuming pork and wine.

The fatwa enjoyed wide currency among Spanish Muslims and Moriscos—Muslims nominally converted to Christianity and their descendants; one of its surviving aljamiado translations was dated at 1564, six decades after it was first issued. The fatwa has been described as the "key theological document" to understand the practice of Spanish Muslims following the Reconquista up to the expulsion of the Moriscos in the early 17th century.

The influence of the Oran fatwa was limited to Spain: Outside the Iberian Peninsula, the predominant opinion upheld the requirements of Islamic law and required Muslims to emigrate, or even choose martyrdom, when the orthodox observance of the religion became impossible.

Islam existed in Spain since the Muslim conquest of the Iberian Peninsula in the eighth century. At the beginning of the twelfth century, the Muslim population in the Iberian Peninsula, called al-Andalus in Arabic, was estimated to number up to 5.5 million, among whom were Arabs, Berbers and indigenous converts. In the next few centuries, as the Christians pushed from the north in a process called the Reconquista, the Muslim population declined. At the end of the fifteenth century, the Reconquista culminated in the fall of Granada, and the total number of Muslims in Spain was estimated to be between 500,000 and 600,000 out of the total Spanish population of 7 to 8 million. Approximately half of the Muslims lived in the former Emirate of Granada, the last independent Muslim state in Spain, which had been annexed to the Crown of Castile. About 20,000 Muslims lived in other territories of Castile, and most of the remainder lived in the territories of the Crown of Aragon.

Prior to the completion of the Reconquista, the defeated Muslims were generally given freedom of religion as terms of their surrender. For example, the Treaty of Granada, which governed the surrender of the emirate, guaranteed a set of rights to the conquered Muslims, including religious tolerance and fair treatment, in return for their capitulation. The increasing occurrences of forced conversion triggered a series of Muslim rebellions in Granada (1499–1501). The rebellions were suppressed, and afterwards the Muslims in Granada were no longer given the rights that had been provided to them by the Treaty of Granada. They were given the choice to remain and accept baptism, to reject baptism and be enslaved or killed, or to be exiled. The option of exile was often not feasible in practice because of the difficulty in uprooting one's family and making the journey to Muslim lands in North Africa, the inability to pay the fee required by the authorities for safe passage, and the general tendency by the authorities to discourage and hinder such an exodus.

Some Muslims, especially those living near the southern coast, took the option of exile, but for most, publicly converting to Christianity while secretly continuing to believe and practise Islam was the only available option for surviving as Muslims. The population converted en masse, and by 1501 the entire Muslim population of Granada was nominally converted to Christianity. The apparent success of Granada's forced conversions triggered a series of edicts and proclamations in 1501 and 1502, which effectively put the Muslims elsewhere in Castile to the same fate. These new converts, along with their descendants, were known by Spanish sources as the Moriscos. As well as having to accept Christianity and abandon the Islamic faith and rituals, they were also pressured to conform to Christian ways, including by attending church, sending their children to be instructed in the Christian doctrine, and partaking of food and beverages forbidden by Islamic law.

Prior to the Oran fatwa, the predominant position of Islamic scholars had been that a Muslim could not stay in a country where rulers made proper religious observance impossible. Therefore, a Muslim's obligation was to leave, when they were able to do so. Even before the systematic forcible conversion, religious leaders had argued that Muslims in Christian territory would be subject to direct and indirect pressure, and preached emigration as a way to protect the religion from erosion. Notably, the contemporary Algerian scholar Ahmad al-Wansharisi, who was considered the leading authority on the subject of Muslims in Spain, wrote in 1491 that emigrating from Christian to Muslim lands was compulsory in almost all circumstances. Further, al-Wansharisi urged severe punishment for the Muslims who remained and predicted that they would temporarily dwell in hell in the afterlife.