The p- form (as opposed to the v of ovre1) is closer to the Latin etymon opera. It is a Latinism that is only attested in Anglo-Norman from the mid-fourteenth century onwards, and almost exclusively in formulaic phrases.
s.
1act, deed:
(
s.xvin; MS: s.xv1
)
L'autor dit quy vostre entente Mettere devez en ta juvente De toutz pecchez vous retrahere Et bonez oouprus user et faire (M.E. And goud werkys use and doo shal ȝe)100.16
(
1347-59
)
En .xxxvj. barels de pych achaté as ditz escomers et tylater, pris .iiij. s. du piece, .vij. l. .iij. s. En .xxiiij. barils de tar achaté pur mesmne l'oepre, pris .iij. s. .ij. d. du piece, .lxxvj. s. En .xxiiij. gallons d'oyle achaté pur mesme le opre, pris .xij. d. du galon, .xxiiij. s.
iii 145 (PRO E101/25/32 m.3)
pur Deu et en opere de charité
1lawfor God and by way of charity (a standard formula ending a legal petition):
[opere] (1341)