The Public Library of Cincinnati and Hamilton County Ohio and various staff or volunteers have browsed the old newspapers in Cincinnati and created an index of obituaries.I'm not sure how far back it goes, but it is being kept up to date with recent entries.
The online browser interface is quite awkward as some of the commands seem to be the style of the old mainframe interface.
Anyway, I did not find an obituary for Otto Alphons Amann, but I did find his widow, I presume, Alberta M. Amann.Look it up in the Cincinnati Enquirer dated 21 Jan 1991 page A9 column 3.
I find it strange that a church would deny records access to descendants of an individual so long after his death.I would agree it made sense during his lifetime and the lifetime of his parents, but surely enough time has passed to allow his own grandchildren the access.Did you clearly state you were the grandchildren?
More ideas, have you yet tried to find the death certificate for Otto Alphons Amann, as parents are listed, if known by the informant.Have you yet tried to find him in the 1910 census?It would be nice to know how and where that census reports him.
The Probate Court Records room on the 10th floor on the 9th street courts building does has a few old birth record books.Somewhat incomplete, so I don't know it he might be there.Also, the downtown library has numerous books published which extracted and organized data from many churches, cemeteries, city or county offices, and court house records (sadly some court house records were destroyed by fires during the 1800s).Just maybe there is something to help you there.