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Abstract Phenoxazines are a successful class of organic photoredox catalysts (PCs) with tunable redox and photophysical properties. Originally, we aimed to realize more reducing phenoxazine PCs through heteroatom core substituted (HetCS) derivatives, while maintaining an efficiently oxidizing PC^·+. However, core modification with thioether or ether functionality to a PC that exhibits photoinduced intramolecular charge transfer (CT) negligibly alters the singlet excited state reduction potential (E_S1°*), while yielding a less oxidizing PC^·+(E_1/2) (E_1/2 = 0.50–0.64 V vs. SCE) compared to the noncore modified PC1(0.68 V vs. SCE). Photophysical characterization of HetCS PCs revealed that increasing electron density on the core of a CT exhibiting PC stabilizes the emissive state and PC^·+, resulting in a relatively unchangedE_S1°* compared to PC1. In contrast, modifying the core of a PC that does not exhibit CT yields a highly reducingE_S1°* (PC3= −2.48 V vs. SCE) compared to its CT equivalent (PC1d= −1.68 V vs. SCE). The impact of PC property on photocatalytic ability was evaluated through organocatalyzed atom transfer radical polymerization (O‐ATRP). HetCS PCs were able to yield poly(methyl methacrylate) with low dispersity and moderate targeted molecular weight as evaluated by initiator efficiency (I*) in DMAc (Ð= 1.20–1.26;I*= 47–57%). Ultimately, this work provides insight into how phenoxazine PC properties are altered through structural modification, which can inform future PC design.