The optical transition properties of trivalent rare earth (RE³⁺) doped luminescent materials have received extensive attention. The Judd–Ofelt theory is an effective tool for exploring the optical transition properties for the 4f–4f transitions of lanthanides. The aim of this work is to discover the effect of Er³⁺ concentration and different Ln³⁺ ions on the Judd–Ofelt parameters in LnOCl:Er³⁺ (Ln = Y, La, Gd) phosphors. Oxychloride LnOCl:Er³⁺ (Ln = Y, La, Gd) phosphors were produced via a single displacement reaction technique. The Judd–Ofelt calculation procedure for RE³⁺ doped powders was modified and then adopted to obtain the Judd–Ofelt parameters of Er³⁺ in the studied phosphors. Meanwhile, a new route for examining the Judd–Ofelt calculation quality was proposed and used. It was found that the Er³⁺ doping concentration slightly affects the optical transition properties of Er³⁺ in YOCl and LaOCl, but greatly affects the optical transition properties in GdOCl. Moreover, it was also found that the optical transition properties of Er³⁺ depend also on Ln³⁺ (Ln = Y, La, Gd) though the crystal structure of these compounds is similar. The Judd–Ofelt parameters of Er³⁺ are the smallest in LaOCl:Er³⁺, medium in YOCl:Er³⁺, but the biggest in GdOCl:Er³⁺ when the doping concentration is the same.