He was a surodnikь of Markos/Marko Kraljević. The only information about the career of Ostoja Rajaković is known from the sepulchral inscription above the painted portrait composition in the arcosolium of the narthex in the Church of the Holy Mother of God Peribleptos in Ōhrid (Prěstavi se rabь bžii Ōstoja Rajaković po guslomь Ugarčikь. i surodnikь kralja Marka. zetь župana Grōpe. Lěto 1379 msca oktōmbria. i. ind. 3. Vasь že mlju bratija moja ljubimaja pročitajušte prostite raba, jako vy možete byti kako ja. a ja kako vy nikoliže). The hypothesis about his origin in Hercegovina from the family of Ugarčići is higly probable, although even the fact, that he was a Greek, cannot be excluded. With regard to the employed word gusle the presumption of Friedrich Ress is, that Ostoja Rajaković was a gusle player in the circle of Markos/Marko Kraljević. His opinion was not accepted in the literature.