Background: Type 2 diabetes mellitus is a serious health concern, as it remains the leading cause of mortality worldwide. Despite the available antidiabetic drugs used in the management of diabetes, the continuous increase in hyperglycemia has prompted growing interest in developing effective, green alternative therapeutic strategies for diabetes mellitus. Hence, this study aimed to synthesize O. paniculosa extract-passivated AuNPs and evaluate their antidiabetic properties. Methods: The plant material was sequentially extracted with n-hexane, dichloromethane (DCM) and 70% ethanol (70% EtOH), after which the resulting extracts were used to synthesize AuNPs that were characterized through various analytical techniques. The antioxidant activity of AuNPs was screened against DPPH, ABTS, metal iron chelating and reducing power. The enzyme inhibitory activity of AuNPs on α-amylase, as well as the ability of AuNPs to glycate protein, was investigated on a BSA-fructose model over a period of 28 days. Results: The FT-IR, pXRD, HR-TEM, STEM-EDS and UV-Vis spectroscopy analyses confirmed the synthesis of O. paniculosa extract-passivated AuNPs bearing faceted NPs and triangular 2D nanoplates morphologies. Despite the poor scavenging activity of AuNPs on DPPH and ABTS, a significant metal-ion chelating potential with IC₅₀ values ranging from 76.7 ± 0.02 μg/mL to 83.3 ± 0.03 μg/mL and a good reduction potential was observed. Furthermore, an inhibition in concentration-dependent α-amylase activity was observed, with IC₅₀ values ranging from 76.7 ± 0.02 μg/mL to 380 ± 0.12 μg/mL, and the formation of AGEs was suppressed, with an inhibitory effect observed over a period of 28 days. Conclusions: The results obtained suggest that O. paniculosa extract-passivated AuNPs possess some potential antioxidant and antidiabetic properties.