Osman Gazi (1258-1324) is the founder Ottoman Principality and Dynasty. Most Ottoman chronicles associate the foundation date of the state with 1299/1300 which is the conquest date of Karacahisar. It is stated Bafeus (Koyunhisar) Battle (1302) won by Osman Gazi against the Byzantines has a significant role in his appearance on the stage of history. Thanks to locations taken after such a battle, the path to İznik and Bursa was opened to Osman Bey. When Osman Gazi died, he was ruling the lands between Eskişehir and Bursa and blockading İznik and Bursa. 

A tahrirat was written to Mâbeyn-i Hümâyûn-ı Mülûkâne Başkitâbet-i Celîlesi of Mektûbî Kalemi of Hüdavendigar Vilayeti. The subject of the correspondence is related to the construction of a madrasah adjacent to the Gazi Mosque in Bilecik

As a result of the research, Keşfi Evvel Defteri of the madrasah and a map of the location have been prepared. According to the inquiry carried out, it was required to purchase a dwelling that cost three thousand kuruş (penny) and then in order to expand the location. It was calculated that the total cost of the construction would be twenty-nine thousand three hundred and eighteen kuruş when the cost of the dwelling was taken into consideration. 

In the Keşif Defteri, there is a sketch of the eastern entrance facade of the madrasah with image drawing and inscription of mürtesem-i amûdî (facade) on it. By providing measurements on the drawing, facade, door and window heights were written next to related parts with a red pen. In the mürtesem-i amûdî sketch of the facade, the height of the facade was drawn beginning from the ground line up to the basement level and from the basement to the eaves. In the centre of the facade, there is a handrail stair leading to the entrance door and it has a symmetric design. 

We understand that the drawing provided together with the resm-i musattahı (plan-section) record of the building is the plan sketch of the madrasah. In the drawing, wall lengths, window and gate openings were written with a red pen and the drawing were designed in this manner. According to the drawing, the madrasah is composed of a rectangular and closed inner courtyard positioned in an east-west direction in the centre and rooms lining around the courtyard. 

Among the sketches, there is a plan on which it is written the Osman Gazi Mosque. The harim section of the mosque has a rectangular plan in a North-south direction, and there is a courtyard in the east-west direction to the west of the sanctuary. In the northern section of the courtyard, there is a square-base minaret adjacent to the main wall of the mosque and a madrasah adjacent to the wall of such minaret. The madrasah is composed of cells constructed side by side with a rectangular plan. With two steps taken upward from the courtyard, it is passed through gezinti, vakithâne and abdesthâne units.

The new madrasah to be constructed has been designed to include the dwelling to be purchased, the western section of the Mosque’s courtyard and the abdesthâne of the existing madrasah.

Ayverdi states that there is no building in Bilecik, which dates back to the period of Sultan Osman Gazi, and the Osman Gazi Mosque in Bilecik dates back to the Sultan Orhan foundation. The Mosque was burnt completely during the fire set by the Greeks invading the region during the War of Independence, together with the old Bilecik town; only its minaret and ruins of the walls have survived to this day. 

On November 3-22, 2008, cleaning and excavation works were carried out around the building by Erdal Mean under the supervision of the Museum directorate. With the findings unearthed during such excavation, data validating the plans in the archive have been obtained. E. Mean excavated by separating the area as A and B sections. Especially the clean-cut block stones unearthed in the excavation of Section A and pipes with lenght of 9 m and extending towards the centre of Section A have revealed that such section is the rectangular planned courtyard. 

During the excavation, rubble foundation ruins and early-period slate grounds were found between the minaret and the northern wall of the courtyard. The minaret of mosque survived intact today and has square bases, a cylinder body and a single balcony. To the South of the minaret, a six-line repair inscription dated 1848 in Gregorian (1265 in the Islamic Calendar) was found, which indicates the period of Sultan Abdulmecid. As a result of findings unearthed during the excavation works in B Section, it has been understood that such section belongs to the square planned mosque. 

The documents have revealed a significant discovery in terms of the architectural history of the Ottoman period; because we do not have any information about the original states of the buildings which were destroyed during the fire, except for the minaret. Our knowledge, apart from the documents obtained in the archive, consists of this for now. The mosque was certainly repaired several times during history, but I think that its original state is the same as the plans I have published due to the sloping and narrow conditions of its land. It has been foreseen that such walls would be removed as a plan for a new madrasah would comprise the courtyard and a part of the existing madrasah. During the excavations, ruins belonging to the courtyard and madrasah walls were found. Accordingly, it is understood that despite the efforts and works made for the construction of the new madrasah, it could not be applied and remained in the Project stage. I am publishing this exciting discovery for the first time, and I feel very happy to introduce it to the scientific world to provide a basis for future studies.