|Through the entrance portal is an enclosed gallery on the south-western wall of which is the famous mehrab of Oljeitu. This has been described by Pope as "the most subtle and perfectly proportioned of the fourteenth century mihrabs". It dates from 1310 and the signature proclaims it to be the work of Badr. The minbar on the right of the picture is said to be contemporary with the mehrab.
The mehrab can be viewed in several sections. The arched area surmounting the two slender columns is filled with elaborate thuluth script set upon a complex pattern of arabesques with an almost infinite variety of foliate patterns. This describes the commissioning of the mehrab.
Above the actual mehrab there is a lintel set with the Shi'ite profession of faith set in plaited kufic.
Golombek and Wilbur have suggested that the function of this room was to provide a secure, sheltered and therefore warm part of the mosque. They draw attention to the discrepancy in dates between those on the mehrab, dated 1310 C.E. and those on the portal, dated 1447 C.E., and suggest that this may be resolved by ascribing two separate dates to the construction of the portal, and suggest that the patron, Imad al-Din may have only carried out repairs, rather than construction. To the right of the mehrab a small doorway leads into the Winter Prayer Gallery, with its curious low-sprung vaults.