Designing and preparing flexible supercapacitor electrode with high specific capacitance and good cycle stability is still a severe challenge. In this study, a sandwich-like nickel oxide/nanoporous nickel/metallic glass (NiO/np-Ni/MG) composite electrode was facilely synthesized by dealloying the Ni45Ti20Zr25Al10 MG ribbons in O2-rich HF solution. Different from the routine free-dealloying, the etching with continuous O2 flow in HF results in the in-situ growth of NiO micro-rod arrays on np-Ni surface. The shape of NiO from an initial flower-like architecture evolves to be the regular hexagonal pillars during the O2-assisted dealloying process. The possible nucleation and growth mechanism of hexagonal NiO is proposed. Moreover, the as-designed sandwich-like structure offers an excellent flexibility of the composite electrode due to the high ductile Ni-based MG interlayer. Meanwhile, the new-type NiO/np-Ni/MG electrodes demonstrate a high specific capacitance (745.3 F/cm3 at 0.5 A/cm3) in KOH solution and excellent cycling stability (92% retention after 6000 cycles at 20 mV/s). The symmetric supercapacitor (SC) devices were successfully assembled by using the dealloyed ribbons and showed a maximum volumetric energy density of about 19.82 mW h/cm3 at a power density of 0.4 W/cm3, and even kept at 8.71 mW h/cm3 when the power density increased to 1.6 W/cm3. The fully charged two SC devices connected in series could light up a red LED bulb for more than 10 min. The novel developed NiO/np-Ni/MG electrodes with good energy storage performance and excellent flexibility show highly promising application potential in wearable energy storage devices.