During the excavations in the south-eastern part of the Upper city of Olbia in 1998 a small fragment of a marble slab with a relief of a Thracian horseman and two lines of a Latin inscription was found. The relief belongs to a group of rather rare reliefs, where a Thracian horseman is depicted hunting a wild boar running under the croup of his horse. The appearance of such slabs in Olbia was connected with Thracian warriors in the Roman garrison there. The inscription can be restored in as follows: - ET COH(ortis) I CILICUM DE/[CIANAE] [EX] VOTO POSUIT. Translation: «-and coh(ort) I Cilicianae De(cianae) (by) promise dedicated». According to this interpretation the inscription can be dated back to AD 249-250. It is the latest epigraphical monument which testifies to the residence of the warriors of the 1st Cilician cohort on the territory of Moesia Inferior and in the Greek centres of the northern coast of the Black Sea. It is also the first epigraphical monument which testified to the residence of the warriors of the 1st Cilician cohort in Olbia and proves that the cohort was awarded the honourable title Deciana. Together with the already known epigraphical monuments this dedication permits to specify some aspects of the history of Olbia in the middle of the 3rd century, first of all the composition of Roman garrison, the date of its withdrawal from the city, penetration and dissemination of Thracian cults there. Roman military administration assigned a certain part to Olbia in the politics of containing the barbarians at the Danubian frontiers of the Empire until the middle of the 3rd century.