Metal-free purely organic phosphors (POPs) are emerging materials for display technologies, solid-state lighting, and chemical sensors. The past decade has seen the promising utility of the El-Sayed rule and heavy atom effects in the design of POPs, and efficient matrix engineering to boost emission efficiencies. However, due to limited contemporary design strategies, the intrinsic spin-orbit coupling (SOC) efficiency of POPs remains low and their emission lifetime is pinned in the millisecond-second regime. Here, we report a universally applicable methodology to synergistically manipulate the main descriptors in SOC - heavy atom effect and orbital angular momentum, assisted by a novel set of natural-transition-orbital-based computation methods to visualize angular momentum descriptors in molecular design. Prototype POPs with efficient room-temperature phosphorescence were designed with SOC efficiencies boosted beyond 102 cm‑1 and lifetime pushed below the millisecond regime. Experimental verification for our novel design rule was conducted through systematic computation-assisted design achieving discrete tuning of heavy atom effects and orbital angular momentum.