Normalized alignment, ρ₀⁽²⁾(n_H, B)/ρ₀⁽²⁾(n_H = 0, B = 0), as a function of the magnetic field strength B for several hydrogen densities n_H, for four levels of O I. From top to bottom: 3p ⁵P₁, 3p ⁵P₂, 3p ⁵P₃, and 3s ⁵S₂. Each curve corresponds to a fixed value of n_H, ranging from 0 to 5 × 10¹⁶ cm⁻³, as labeled. The plots illustrate how both elastic collisions with neutral hydrogen and the Hanle effect contribute to the depolarization of each level.