To investigate the genetic regulatory mechanism of supplementary feeding on muscle development in Oula sheep, we employed transcriptomic analysis to explore the differentially expressed genes (DEGs) in the longissimus dorsi muscle of Oula sheep at different ages under conditions of supplementary feeding and non-supplementary feeding, as well as the significantly enriched Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) signaling pathways of DEGs. Moreover, by combining with the method of weighted gene co-expression network analysis, we screened for the potential hub genes that might play crucial roles. The results demonstrated that the CD4 and ICAM1 genes and the PI3K-Akt signaling pathway might exert important functions during the lamb stage. At the growth stage, the AGL, PGM2L1, PRKAA2, NEDD4, and GBE1 genes might serve as core genes to regulate the growth of skeletal muscle in Oula sheep after supplementary feeding through signaling pathways such as starch and sucrose metabolism and insulin signaling pathway. This outcome provides a molecular-level interpretation of the regulatory mechanism of supplementary feeding on muscle growth and development in Oula sheep at different ages, offering a theoretical basis for the further improvement of the meat quality of Oula sheep and the enhancement of the quality of livestock products in the Qinghai–Tibet Plateau region.