TDMA-based Medium Access Control (MAC) protocols for ad hoc networks achieve good performances at high loads and high network densities. Nevertheless, their slot assignment techniques infer inefficient and unfair resources exploitation due to free slots appearance and slot assignment regardless the effective needs. This paper tackles these disadvantages by performing a slot-by-slot reservation mechanism. It results a new MAC protocol, called OSTR (One shot Slot TDMA-based Reservation). OSTR operates in a distributed and slotted environment. It is a topology-based and dynamic frame size-based protocol. OSTR leads to a slower frame size increase. Experimental results show that OSTR achieves a shorter frame size even with high densities inferring a better channel utilization.