The Oldenburg State Archive holds a lot of old testaments. There are,
however, no alphabetical registers. You have to look for the
'urkundenbuecher', where all kind of contracts were documented (testaments,
sales contracts, marriage contracts etc.) These 'urkundenbuecher' are written
in chronolocical order.
My approach: Once you know the death date, go through these books after that
date. If there was a testament, it should have been published shortly after
the death of the person.
The Urkundenbuecher are organized by 'Gerichtsbezirk' . If have no experience
with Cloppenburg, but have found a lot in Wildeshausen, Westerstede,
Oldenburg etc.
Gerold Diers