West Ham had already secured top spot and an automatic last-16 berth while runners-up Dinamo will face one of the third-placed teams dropping down from the Champions League.
Orsic again shone against Premier League opposition in Europe's second-tier competition after netting a hat-trick in Dinamo's memorable 3-2 aggregate win over Tottenham Hotspur in last season's knockout rounds.
The winger beat West Ham's reserve goalkeeper Alphonse Areola with a dipping shot from the edge of the penalty area into the top corner, delighting a handful of away fans at London stadium.
Areola did well to keep out two long-range efforts from Luka Ivanusec and another by Orsic as Dinamo dictated the game and were rarely troubled by the home side.
Earlier on Thursday, Napoli secured a playoff berth with a thrilling 3-2 home win over Leicester City in Group C which sent the Premier League side from first to third in the standings and out of the competition. Spartak Moscow topped the group with a 1-0 victory at Legia Warsaw.