Prabhjot scored his team's second, sixth and seventh goals - the latter two in the last ten minutes of the one- sided game between two former champions, while Gagandeep Singh scored the first and third goals from the field.
Gurjinder Singh struck a rebound home off a long corner from the left for Indian Oil's fifth goal before captain and ex-international Deepak Thakur rounded off the tally at the fag end of the match with a close-range effort.
The huge defeat bundled Army XI out of the competition as they had lost to local outfit Central Railway two days ago in the three-team group.
Central and Indian Oil will fight it out on December 2 to decide who, from among them, will enter the semis.
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In another all-Mumbai game in Pool D, local super league runners-up Western Railway edged out Bombay Republicans 3-2 after holding a 2-0 half-time lead.
Nitin Tigga, Rajan Kandulna and Amit Rohidas scored for the railmen while for Republicans, who ended their campaign with their second loss in the group, Navin Kheretkar and Anurag Sharma were the goal getters.
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