Synchrotron X-ray diffraction measurements of holmium were conducted up to 327 GPa using toroidal diamond-anvil cells, revealing a complete structural sequence of $\mathrm{hcp}\to \text{Sm-type}\to \mathrm{dhcp}\to \mathrm{dfcc}\to oF16\to oF8$. The $oF16\to oF8$ transition initiates around 257 GPa, approximately 60 GPa higher than that in dysprosium, and these two phases coexist up to the highest pressure studied. The transition proceeds sluggishly without a measurable volume discontinuity, indicating an anisotropic lattice rearrangement rather than abrupt $4f$ electron delocalization. The $oF8$ phase exhibits the highest incompressibility among the observed phases, completing the systematic evolution of the heavy lanthanides and underscoring the unified $oF16\to oF8$ sequence in $4f$ and $5f$ metals. First-principles calculations of phonon dispersion and enthalpy indicate that both $oF16$ and $oF8$ are dynamically stable at 325 GPa and remain nearly degenerate over a broad pressure range, consistent with the experimentally observed phase coexistence. Based on comparative analysis, this work also discusses potential method optimizations for high-pressure structural studies of rare-earth metals using toroidal diamond-anvil cells, including calibrant and gasket configurations that could improve data quality in future multi-megabar investigations.

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• Crystal structure
• Equations of state
• High-pressure studies
• Phase transitions
• Lanthanides
• Pressure techniques