-oyl
noun combining form
-o·yl
əwəl,
ə(ˌ)wil,
əˌwēl
plural -s
: acid radical —used in the system of nomenclature adopted by the International Union of Pure and Applied Chemistry in names of radicals derived from acids whose names end in -oic
decanoyl
and also most other organic acids oleoylphthaloyl
compare -yl 
Word History
Etymology
International Scientific Vocabulary -o- (as in -oic) + -yl