How about O_CREAT_DIRECTORY?

O_CREAT is seven chars :) It also didn't arrive until 2.10BSD or so, which also added longer identifiers like O_APPEND and O_RDONLY. It's obviously named for the older creat() syscall.

For that matter, creat is also not six characters. Even back in the PDP-7 Unix that is as early as the tuhs website has, there are some six character syscall names, like rename. The linker being used on that PDP-7 seems to have had a "truncates to 8 chars" limit, as symbols like ttyrestart come out as ttyresta in the linker map; the convention seems to have been a leading '.' on syscall entrypoints, but that still leaves 7 chars. So it seems to me that while you're right that some older systems definitely did have very short limits, at the point when Thompson and Ritchie were defining unix syscall names, the choice of "creat" over "create" was not imposed by a technical limit but was rather "we like and are used to abbreviated names".