Top panels: run of line strengths (relative to Hβ = 100) of three collisionally excited oxygen lines, O iv] λ 1402 Å, [O iii] λ 5007 Å, and [O iv] λ 26 μm, with stellar temperature for three α = 3 model sequences, viz. Z = Z_GD (left), Z = Z_GD/10 (middle), and Z = Z_GD/100 (right). The evolution is only shown until maximum T_eff in order to avoid confusion. Note the different ordinate ranges. Bottom panels: line strength ratios L_eq/L_dyn. The small scatter seen for the λ 1402 Å ratio in the metal-rich model is due to numerics.