Exiled pretenders were a reality of just about every throne in the middle ages. Prior to the fall there had been a 'Prince Osman' living in Constantinople (and the treat by Constantine XI of releasing him from house arrest was one pretext Mehmed used to justify his attack). One of the little known pretenders of history is the case of 'Callixtus Ottomanus' a mysterious figure who floated at the periphery of events for around fifteen years. He first is mentioned in June 1456 when Francesco Sforza, Duke of Milan, received a report of a young boy supposedly brother to Sultan Mehmed and said to have been entrusted to a Latin Knight, one Giovanni Torcello by the Grand Vizier, Halil Candarli. The boy had come into the hands of agents of Pope Calixtus III, and reached Venice that spring.
From Venice he was taken to the Apennine fortress of Spoleto. In a work appearing in 1458, Candarli Halil Pasha is credited with a role in sending the boy to Italy. The truth or otherwise of this remains unknowable, but it has a certain amount of logic to it. Candarli was a court maneuverer par excellence. It seems very likely that he had tried (and largely failed) to make Mehmed his puppet and so perhaps saw a pretender in reserve as a prudent fallback. This is, of course, baseless speculation but the boy clearly arrived with some sort of provenance.
The European rulers through whose hands he moved over a two decade period seem to have made little real effort to press the putative claim to the Ottoman throne. This is perhaps not surprising - their focus was to reinstall the Byzantine heirs. In that regard the existence of a pretender was a little more than a negotiation counter or - should military events go their way - a tame-Turk to pop on the throne in Bursa once the dust had settled. Bayezid Osman remained in Spoleto until 1459, when - having been christened Callixtus Ottomanus - he was taken by Pope Pius II on his progress through Italy, culminating in the congress of Mantua, at which a crusade to recover Constantinople was proclaimed.
Bottom Right of picture: Callixtus Ottomanus
In 1464 the Pope again paraded his charge in public: he had the boy, now sixteen, bid farewell to the fleet setting off from Ancona against the Ottomans, a scene commemorated in a fresco by Pinturicchio in Duomo of Siena. Then disaster struck, the Pope died before the fleet could depart and the whole Crusade was put on ice. The next year Bayezid Osman / Callixtus Ottomanus, was in Venice - the new hotspot for Crusade- before moving on to the court of King Corvinus in Buda. The long war failed to make progress towards unseating Mehmed and by 1473 Bayezid Osman was at the Viennese court of the Holy Roman Emperor Frederick III who was famously fascinated with Turkish style - particularly dress. Bayezid Osman toured around his domains in the imperial retinue like an exotic beast. In 1474 he married an Austrian noblewoman, and subsequently disappeared from history's view.
We have no certainty over the veracity of his claim. Mehmed had supposedly had his brothers all killed long before 1456 but it is possible that the far-seeing Candarli smuggled one away. It may be more likely that Candarli simply invented the claim - one sweep of his signature was all it would take once Murad was dead - to bestow the lineage of Osman upon a suitably pliable child and place him into the hands of Mehmed's enemies. Deep down, perhaps the Christian kings suspected the dubiousness of the claim. In any case a better pretender soon came along once Mehmed died in 1481.
Prince Cem lost the power struggle for inheritance with his brother Bayezid. In 1482 he fell into the hands of the Knights of Rhodes. Here was a candidate for the Ottoman throne who's claim was indisputable. There is no record of Callixtus Ottomanus after 1474. Perhaps he simply died or perhaps he settled down into quiet domestic bliss with his Austrian wife and was thankful for it - the life of a pretender was a precarious existence as Prince Cem discovered in 1495 when, shortly after transferring to the care of the French King, he died - poisoned, rumour had it, at the behest of his brother by a slow acting poison some say applied on a straight razor by his Turkish barber, others point the finger at the Borgias.