Lithium cobalt oxide (LiCoO₂) is a widely used cathode material whose mechanical behavior under pressure is increasingly relevant to cell manufacturing and extreme-environment applications. Yet a coordinated understanding of how hydrostatic pressure simultaneously affects its structural, elastic, and vibrational properties has been lacking. Here, we employ first-principles DFT-GGA-PBE calculations to investigate the structure, elastic constants and moduli, and phonon band structure of $R\overline{3}m$ LiCoO₂ under hydrostatic pressure from 0 to 50 GPa. The in-plane and interlayer lattice parameters contract anisotropically, with the c-axis showing greater compliance; our computed $c/a$ evolution reproduces the experimentally observed decreasing trend, lying systematically ∼2% below the convergent experimental datasets over the experimentally covered pressure range, with the deviation narrowing toward the highest experimentally covered pressures. All six independent pressure-corrected elastic constants increase monotonically and satisfy the Born stability criteria over the full pressure range. The polycrystalline bulk modulus more than doubles from 0 to 50 GPa, while the Young’s and shear moduli increase by 27–37%, with zero-pressure values ($B_{\mathrm{H}}=144.76$ GPa, $E_{\mathrm{H}}=214.65$ GPa, $G_{\mathrm{H}}=85.66$ GPa) in good agreement with pulse–echo and nanoindentation measurements. Phonon frequencies exhibit a systematic blueshift, with the highest-frequency optical modes hardening at an average rate of 0.102 THz/GPa, and remain real across the entire Brillouin zone, confirming the dynamical stability of the $R\overline{3}m$ phase. The correlated hardening of elastic moduli and phonon frequencies is consistent with pressure-induced bond shortening and the associated strengthening of the interatomic interactions, as evidenced by the monotonic contraction of the Co–O and Li–O bonds (by 4.1% and 9.7%, respectively, over the full pressure range). These results provide a quantitative benchmark for the mechanical behavior of layered oxide cathodes under pressure and offer a reference for related materials.