buckstix,
A really cool piece of history. While I can't seem to find anything on the .280 OKH per se, there are references that O'Neil tried the .375 case and the .30 Newton case before deciding the '06 case was the best with available powders.
See Simmons' book WILDCAT CARTRIDGES and Hagel's OKH CARTRIDGES in the #4 Handloader.
The load data you posted appears to be for the .285 OKH and not a bigger case according to the publications listed above.
Please keep us posted on your research.
Good shooting.
