The use of essential oils (EO's) as feed additive has been explored as tool to improve fish growth and health. The aim of this study was to evaluate the oregano essential oil (OEO) (Origanum vulgare) dietary supplementation for the catfish Lophiosilurus alexandri. Four diets supplemented with different OEO levels (0.0, 1.0, 2.0 and 4.0 g kg−1) were evaluated in a feed trial conducted with fish of 15.7 ± 1.8 g and 9.5 ± 0.45 cm for 13 weeks. Were evaluated growth performance, biochemical parameters, intestinal histomorphometry, whole‐body composition and muscle growth. Polynomial analysis showed that weight gain, length gain and specific growth rate were improved in fish fed with OEO supplemented diets, being 2.37, 2.56 and 2.33 g kg−1 the respective levels that maximize these variables. Plasma glucose, cholesterol, triglycerides, LDL and HDL increased with OEO levels between 1.80–2.47 g kg−1. Intestinal histomorphometry was not affected by OEO. Body protein content and hyperplasic muscle fibres were maximized with 2.10 and 2.25 g kg−1 OEO, respectively. Body moisture, total lipids, ash and hypertrophic muscle fibres were not affected by OEO. In conclusion, dietary supplementation with 2.10–2.56 g kg−1 OEO improves growth performance, body protein retention and muscle growth of L. alexandri.