Recently, Duan et al. reported a B,N-doped pentacene emitter, mDBIC, that emits in the deep blue (lPL = 426 nm, FWHM = 26 nm) region in 2 wt% doped mCP film and was used as the terminal emitter in a HF device that showed excellent performance at the deep blue color point (lEL = 431 nm, FWHM = 42 nm, EQEmax = 13.5%, CIEy = 0.05). Pushing the emission further into the deep blue in MR-TADF compounds remains exceedingly difficult. The B,N-doped heptacene system α-3BNOH (lPL = 390 nm, ΔEST = 0.30 eV, τd = 0.45 μs, THF) that we previously reported represents a rare example of a MR-TADF purple emitter as does the B,O-doped triangulene emitter, DOBNA (lPL = 398 nm, ΔEST = 0.18 eV, τd = 66 μs, 1wt% in PMMA). Building on our recent efforts to design linearly extended MR-TADF acene emitters, here we report a boron-, nitrogen-, and oxygen-doped polycyclic aromatic hydrocarbon that contains nine annulated sixmembered rings, NOBNacene, (Figure ). Despite the apparent large conjugation length, maintaining a regioregularity of the heteroatoms, and using oxygen donors rather than more electron-donating nitrogen atoms results in a deep blue emission in this system. This compound represents arguably the first example of an MR-TADF emitter possessing both electronically distinct donor (N and O) and acceptor groups (two distinct B atoms). In 1.5 wt% TSPO1 doped films, NOBNacene shows narrowband emission with a lPL of 410 nm, a FWHM of 38 nm, a high ΦPL of 71%, and a thermally activated delayed fluorescence with a td of 1.18 ms. The ΔEST was measured to be 0.30 eV. The corresponding deep blue OLED shows outstanding performance with a maximum external quantum efficiency (EQEmax) of 8.5% at a peak electroluminescence (lEL) of 409 nm (FWHM of 37 nm), with corresponding to Commission Internationale de l'Éclairage coordinates of (0.173, 0.055), very close to the BT.2020 requirement for the blue pixel of (0.131, 0.046). 25 NOBNacene combines the benefits of high efficiency, narrow near-UV emission, and high color purity, representing a promising emitter design approach to high-performance deep blue OLEDs. Single crystals of NOBNacene were obtained by slow evaporation of a saturated solution of the compound in THF over several days at room temperature. The crystal structure is shown in Figures .