A porous carbon was synthesized by the combination of freeze-drying and CO2 activation from starch. Nonthermal plasma was employed to quickly produce oxygen functional groups on a porous carbon surface. The plasma treatment has a negligible effect on the textural properties of the porous carbon. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses suggested that the plasma treatment significantly increased the amount and promoted the evolution of oxygen groups on surface. The unique pore structure of porous carbon was proven favorable to effective oxygen loading. The elemental mercury (Hg-0) adsorption ability of the oxygen enriched porous carbon was tested. The results indicated that the oxygen-rich porous carbon constitutes an effective sorbent for Hg-0 removal. The excellent textural properties, surface atomic oxygen concentration, and the type of oxygen group are the three key factors for realizing high Hg-0 removal performance. Density functional calculations were performed to understand the effect of oxygen groups on Hg-0 adsorption. Carbonyl and ester groups are beneficial for Hg-0 adsorption, whereas epoxy, carboxyl, and hydroxyl groups inhibit Hg-0 adsorption. Plasma treatment enhances Hg-0 adsorption by increasing the amount of ester and carbonyl groups on surface.