Afterglow emissions from long-lived excited states, such as room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), are highly promising in the fields of display, bioimaging, and data security. However, it is challenging to achieve a single material that simultaneously exhibits both RTP and TADF properties with their relative strengths changed in a tunable manner. Inorganic phosphors or organic compounds have been widely explored for afterglow materials, but they often require expensive rare-earth metals, high reaction temperatures, or complicated synthetic processes. Herein, we propose a new design strategy, where the singlet-triplet energy splitting (∆EST) is tuned by varying the degree of oxidation, i.e., the ratio of oxygenated carbon to sp2 carbon (= OC) in graphene quantum dots (GQDs)/graphene oxide quantum dots (GOQDs). A series of GQDs is prepared to have various OC values ranging from 4.6% to 59.6%, by which ∆EST was changed from 0.37 eV to 0.13 eV, leading to a dramatic afterglow transition from RTP to TADF. Being incorporated into boron oxynitride matrix material, the low oxidized (OC = 4.6%) GQD exhibits RTP lifetime as long as 783 ms, and the highly oxidized (OC = 59.6%) GOQD exhibits TADF lifetime as long as 125 ms. Collective photoluminescence measurements and photophysical kinetics regarding each system reveal that OC is responsible for the transition of afterglow characteristics. In addition, the time-dependent density functional theory elucidates the variation in the afterglow properties are attributed in part to the distorted molecular geometry and the change of spin-orbit coupling matrix element value by an increment of oxygen functional groups. Finally, we demonstrate anti-counterfeiting and multilevel information security, through the long-lived RTP and TADF properties from the oxidation-controlled GQDs, illustrating the immense potential in various fields.