Chemically modified graphene, such as oxo-functionalized graphene (oxo-G), has received considerable interests for electronic, optoelectronic, biological and chemical sensing applications due to its tunable bandgap, diverse luminescence behaviors and the ability to modify carbon atoms covalently and non-covalently. In contrast to pristine graphene with carbon arranged in a two-dimensional hexagonal lattice, oxo-G consists of abundant sp3 hybridized carbon atoms due to covalent bonds of carbon with oxo-groups, mainly hydroxyl and epoxy groups. The existence of surface oxo-groups has profound impacts on improving its hydrophilicity, chemical reactivity, catalytic activity, and optical properties, while the effect is detrimental for the electrical conductivity. The defunctionalization of oxo-G by chemical reduction or thermal annealing leads to a certain type of graphene (G), termed as reduced oxo-G (r-oxo-G). So far, the majority of studies on oxo-G based materials focused on optimizing preparation and reduction methods, understanding preparation protocols and reduction mechanism, and various applications. Deep knowledge about electrical properties and the chemical structure of the reduced oxo-G (r-oxo-G) based materials themselves is still lacking. In detail, no systematic study involves room-temperature transport perfromances of monolayer graphene derived from oxo-G. Currently reported mobility or resistance values were determined from multilayer thin films of the r-oxo-G related materials with unknown thickness and differential quality. It does not make sense for comparing these results obtained from non-standard transport measurements and test conditions. In addition, the structure evolution of the oxo-G remains ambiguous during the thermal annealing treatment. The thesis presented here is dedicated to study the problems mentioned above by using oxo-G and r-oxo-G with various densities of defects as main research objects. Firstly, substrate effects on the room-temperature electrical transport of monolayer reduced oxo-G with defects of about 0.5% (0.5%G) were studied. The results demonstrated that the monolayer 0.5%G on a hexagonal boron nitride (h-BN) substrate exhibited lighter p-doping and smaller hysteresis than on a SiO2 substrate due to less trapped molecules induced by the h-BN buffer layer. Then, the relation between densities of defects in the range of 0.2% and 1.5% and transport properties was quantitatively investigated. The defects divided graphene domains into isolated small islands with a distance between nearest two defects lower than 3 nm. Therefore, the mobility values of charge carriers of graphene with densities of defects between 0.2% and 1.5%, changed from 0.3 cm2 V−1 s−1 and 33.2 cm2 V−1 s−1. Finally, the structural evolution and electrical performances of the monolayer oxo-G during the thermal decomposition process were studied. The defect structures including holes and bilayer C-C sp3-patches induced by thermal disproportion of the monolayer oxo-G were demonstrated. Scanning tunnelling spectroscopy revealed a semiconducting behavior of the nanometer-sized sp3 bilayer structures with a bandgap of ~ 0.4 eV. These fundamental studies can help to better understand oxo-functionalized graphene derivatives and promote more extensive applications.