This study aimed to functionalize the surface of activated carbon, and thus render the surface more hydrophilic and reactive. To attain this goal, sequential surface functionalization was carried out using (i) oxidation (pre-activation) and (ii) secondary functionalization. The carbon surface was pre-activated in an autoclave via solvothermal oxidation (i.e., wet oxidation) with nitric acid. Alternatively, plasma-assisted oxidation with a mixture of argon and oxygen (i.e., dry oxidation) was employed. A subsequent step included the reduction in formed carbonyl groups with LiAlH4. Following that, secondary functionalization was performed with 3-(aminopropyl)trimethoxysilane (APTMS) or (3-glycidyloxypropyl)trimethoxysilane (GPTMS), respectively. Changes in the surface composition of carbon after functionalization and morphology were examined by X-ray photoelectron spectroscopy, ATR-FTIR spectroscopy, and scanning electron microscopy. Oxidized carbon samples were successfully modified at their surfaces with APMTS and GPTMS, yielding Si content of 3.2 at. % and 1.9 at. % for wet-oxidized carbon and 5.1 at. % and 2.8 at. % for dry-oxidized carbon, respectively.