Oxygen-bridged boron (BO) derivatives, 3,11-di(9H-carbazol-9-yl)-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (p-BO-2Cz) and 2,12-di(9H-carbazol-9-yl)-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (m-BO-2Cz), were successfully developed as n-type (electron transport type) hosts with thermally activated delayed fluorescence (TADF) characteristics. These hosts were created by introducing an electron-donating carbazole unit at the meta- and para-positions of the boron in the BO core for p-BO-2Cz and m-BO-2Cz, respectively. Notably, p-BO-2Cz exhibited a higher triplet energy in solid state than m-BO-2Cz, making it suitable for various device applications. In terms of device performance, p-BO-2Cz demonstrated high external quantum efficiency (EQE) of 24.0% and 28.1% in blue and green phosphorescent devices, respectively. On the other hand, m-BO-2Cz exhibited a high EQE of 26.5% only in the green phosphorescent device. These results emphasize the significance of the carbazole substitution position in the BO core, as it plays a critical role in determining the performance of phosphorescent devices. Overall, the introduction of carbazole-modified BO derivatives as electron transport and TADF-type hosts in mixed host systems was proven to be highly effective for PhOLEDs.