Oils first used on some paintings by Bridget Riley, for example (2,3)
I believe the answer is:
'paintings by bridget riley for example' is the definition.
'oils first used on some' is the wordplay.
'first' says to take the initial letters.
'used on' is a charade indicator (letters next to each other).
'some' becomes 'part'.
The first letter of 'oils' is 'o'.
(Other definitions for op art that I've seen before include "Illusory graphic style" , "sort of painting" , "Overwhelm" , "It may give the impression of movement" , "abstract works at Tate Modern?" .)