Oxygen carriers play a key role in establishing a successful chemical looping operation. Its function in transferring oxygen and heat is crucial in the production of desired products. In the light of the commonly studied chemical looping combustion, many studies have been focusing on the reduction of Fe2O3 to Fe3O4 or equivalent. However, the level of reduction in other emerging chemical looping processes, such as chemical looping gasification, reforming, and water splitting, may be higher. The main cause is the need to control the oxygen transfer between different reactors in order to maximize the desired gaseous products. This can in turn cause various performance issues, such as decreased fluidization performance and compromised mechanical strength. Therefore, it is necessary to investigate the properties of oxygen carriers even in relation to reduction of Fe3O4 to FeO and even to Fe. The aim of this work is to present the physical properties of two iron-based oxygen carriers, namely ilmenite ore and LD slag, at higher reduction degrees. The evaluated properties include fluidization performance and attrition rate. The results suggest that the risk for bed defluidization is increased at high reduction degrees due to the formation of agglomerates comprising iron oxide phases with low oxidation states, such as FeO or Fe. Furthermore, the attrition rate of oxygen carriers may be altered at high reduction degrees depending on the type of the materials. These findings are useful for any chemical looping processes, especially those that involve high reduction degrees of oxygen carriers.