So, Ottoman succession is an extremely confusing nuanced evolution spanning six centuries. In 1261, the creation of the Ottoman polity, the Ottoman state used unigeniture to determine the new leader of the polity after the death of a sultan. As you can imagine, when a sultan has a dozen heirs capable of assuming control of the throne, violence occurred in the capital and surrounding areas without fail. With the death of a sultan, it benefitted the possible heirs who were closer geographically to the capital(which the Ottomans did not truly have until 1453) because they would hear about the death of the sultan earlier and thus be able to raise a stronger larger army more quickly than the heirs who did not hear about the death for many more weeks or months.

This caused obvious problems within the empire, how could an empire grow and spread if each time the leader died the state devolved into civil war for weeks, months, or even years while the heirs duked it out in intra-state conflicts? The short answer is that it could not. Pre-interregnum, the Ottoman state would often marry outside of the state into eastern Europe to solidify allies and create economic windfalls for themselves. Although economically the Ottoman State prospered in the 14th century, politically they became weakened by the external political influences. Post-interregnum, marriage took on a new concept in the Ottoman sultanate. No longer were possible heirs born in wedlock to external wives. Sultanic marriages become celibate, and the Sultan bred only with his royal concubines so as to prevent external powers from exerting influence on the princes of the state.

Now, with only internal princes having access to the throne, and harem politics not yet influencing the princes, the first sultan to commit fratricide started an evolving chain of fratricide and set the foundations for primogeniture. When Sultan Mehmed II ascended to the throne for the first time in 1444 he was only 13 years old. He deposed himself and had his father, Murad II, come back to lead the Ottoman armies against the Hungarians and Polish at the battle of Varna in the same year. Mehmed II again took control of the Ottoman throne in 1451, at the age of 21. This time, determined to seize and consolidate power in the sultanate, he preemptively committed fraticide so he would not be challenged in his right to rule the Ottoman Empire.

Succession shifted again in 1520 after Suleiman the Magnificent came to power. Considered to lead during the Ottoman golden age, Suleiman radically shifted the ideas of marriage, external influences, and succession. For the first time since Interregnum a sultan married and copulated with a former slave and concubine. Unlike the sultans before him, he allowed Hurrem Sultan and her offspring to stay at the courts, as normally the offspring and the mother would be banished to distant lands and only brought back if he was deemed to be the heir apparent.

This basically gave Hurrem unmatched political power insofar as she raised the heir to the Ottoman throne while still living in the imperial harem as Valide Sultan, or rather the queen mother. This led to a short period of primogeniture ending two centuries of unigeniture combat. The practice of primogeniture would continue into the 17th century until agnatic succession took over under Osman II after he allowed his brothers to live and avoid spending their life in the Kafes, or golden cages.

Sources:

Goffman, Daniel. The Ottoman Empire and Early Modern Europe. Cambridge Press

Lessons from the Ottoman Harem on Culture, Religion, and Wars Murat Iyigun Economic Development and Cultural Change , Vol. 61, No. 4 (July 2013) , pp. 693-730

OTTOMAN HISTORIOGRAPHY AND THE LITERATURE OF "DECLINE" OF THE SIXTEENTH AND SEVENTEENTH CENTURIES DOUGLAS A. HOWARD Journal of Asian History , Vol. 22, No. 1 (1988) , pp. 52-77