Oklahoma doesn't publish their vital record dates. You may be able to verify that he died there through the Southwest Oklahoma Genealogy Society, www.sirinet.net/~lgarris/swogs/
. They also might be able to look up his obituary.
You can contact the Okalhoma State Dept of Health, www.ok.gov/health/Birth_and_Death_Certificates/
and apply for a copy of his death certificate. There are restrictions which are explained on their web site. Successful Searching!