No, the Lexar Professional CFexpress Type A Gold 256GB is not compatible with the OM System OM-5.
OM System OM-5 has an SD-only card slot (no CFexpress support of any type). Its own spec page confirms only SD/SDHC/SDXC card compatibility — CFexpress Type A uses a physically different connector and cannot be inserted into an SD slot.
What works instead
See all compatible memory card for the Lexar Professional CFexpress Type A Gold 256GB →“SD Memory Card (SDHC, SDXC, UHS-I, UHS-II compatible)”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)