In summary, a weak electron acceptor CCO-α with a rigid polycyclic structure is generated by ring-closure in carbazole-benzoyl framework with an oxygen linkage. And the combination of CCO-α with electron donor SXAC furnish a series of robust blue and deep-blue delayed fluorescence materials (CCO-1, CCO-2 and CCO-3). The ring-closed structure by oxygen atom bridge has no contribution to SOC, but results in higher rigidity and planarity, better conjugation and lower electron-withdrawing ability and can influence excitedstate energy levels of the molecules, leading to reduced ΔEsts. The introduction of tert-butyl group(s) can suppress tight molecular packing and weaken intermolecular interaction, beneficial for acquiring blue emissions in solid state. In comparison with the control material CBP-1 with an unclosed carbazole-benzoyl acceptor, CCO-1, CCO-2 and CCO-3 own improved structural rigidity and weakened D-A interactions, and exhibit faster radiative transition and RISC process with larger krs and kRISCs. In consequence, they emit stronger and bluer delayed fluorescence with shorter τDFs and higher ΦPLs reaching 99%. Moreover, the improved molecular planarity of CCO-α promotes horizontal transition dipole orientation of the materials, leading to higher Θ//s of 93.6% and thus remarkable ηouts of 43.9%. Owing to these distinguished merits, they function outstandingly as emitters in OLEDs, providing blue (CIEx,y = 0.14, 0.18) and deep-blue (CIEx,y = 0.14, 0.15) lights with record-beating ηexts of 43.4% and 41.3%, respectively. And small efficiency rolloffs are achieved by suppressing SSA and TTA with accelerated radiative transition and RISC process. Beside, by utilizing CCO-2 as sensitizer for MR-TADF dopants, high-performance deep-blue (CIEx,y = 0.14, 0.10) and green (CIEx,y = 0.32, 0.64) HF OLEDs are realized with state-of-the-art ηexts of 32.5% and 37.6%, respectively. The stability of these HF OLEDs is also improved, with the LT50 at 100 cd m -2 initial luminance of 1099 h for deep-blue HF device and 34669 h for green one. Thus, we believe that these tailor-made blue and deep-blue delayed fluorescence materials are promising candidats as emitters and sensitizers for the fabrication of high-performance OLEDs, and the presented design strategy and structure-property insights are beneficial for the exploration of luminescent materials for display applications.