Carnosol, a diterpene derived from rosemary, has recently emerged as a potential strategy to counteract skeletal muscle loss associated with aging, during diseases or sedentary lifestyle. It may protect skeletal muscle cells from inflammation, oxidative damage, and atrophy through multiple mechanisms. Here, to investigate the structure–activity relationship of carnosol, both natural and synthetic derivatives were evaluated for their ability to reduce oxidative damages and to enhance skeletal muscle hypertrophy and function. The natural analogues rosmanol and isorosmanol were selected for their conserved hydroxyl groups at positions C-11 and C-12, while differing in the B-ring moiety. In parallel, dimethylcarnosol and dimethylisorosmanol, featuring methoxy substitutions at C-11 and C-12, were used to assess the functional importance of these hydroxyl groups. In two oxidative damage assays, carnosol, rosmanol, and isorosmanol showed similar antioxidant activity, reducing lipid peroxidation accumulation in post-mortem mouse skeletal muscle tissue and phosphorylation of H2AX, a DNA damage marker, in human skeletal muscle cells. Carnosol and isorosmanol, but not rosmanol, promoted myotube hypertrophy and suppressed the E3 ubiquitine ligase MuRF1 in human skeletal muscle cells, while methoxylated derivatives lacked both antioxidant and hypertrophic effects. The functional efficacy of selected compounds was further evaluated in zebrafish larvae. Carnosol improved locomotion, increased slow myosin heavy chain positive fibers, and downregulated E3 ubiquitin ligases. Isorosmanol also enhanced locomotor performance, whereas its methoxylated analogue showed no effect. The obtained results highlighted the role of specific structural elements, conserved hydroxyl groups at C-11/C-12 and a 20,7-lactone moiety, in mediating the muscle-protective effects of carnosol analogues.