Do you mean something like GnuStep? There never was a version of real "OpenStep" that ran on 68k Macs; there was NeXT/OpenStep for 68k NeXT workstations, and variations of OpenStep ranging from a full OS (IE, "OpenStep for Mach" that ran on a small selection of x86, SPARC, and HP PA-RISC hardware) to some application/API-level versions that ran on top of Solaris and Windows NT. (And of course eventually the whole thing morphed into "Rhapsody", and from there into OS X.)

In principle at least you might be able to get GnuStep running on a 68k Mac using NetBSD as the base OS, but it's gonna be a long slog for not a lot of returns.