Using natural orbitals computed as the eigenstates of the virtual-virtual block of the state density matrix instead of the canonical Hartree-Fock molecular orbitals results in smaller errors when the same fraction of virtual orbitals is frozen . However, the direct application of this strategy to open-shell references, as needed in EOM-SF calculations, leads to much larger errors that in the case of closed-shell references. We analyzed the applicability of the Benchmark calculations illustrated that OSFNO delivers robust performance, similarly to the original FNO scheme in the case of closed-shell references. A typical OSFNO population cutoff of 99% leads to the errors in singlet-triplet gaps of 130 cm -1 for same-center diradicals, ∼10-60 cm -1 for benzynes, 20-50 cm -1 for dehydro-meta-xylylenes, and <18 cm -1 for dicopper SMMs. For SMMs, this approach reduces the virtual space by ∼22% for cc-pVDZ and by ∼46% for cc-pVTZ. Using OSFNO has enabled the EOM-SF-CCSD calculations with more than 1,000 basis functions on a single node. Finally, this approximation has a negligible impact on the spin-orbit coupling constants, enabling calculations of magnetic properties in large systems.