The third win in the four-team group helped Indian Oil top the pool with nine points.
Indian Oil recovered after conceding a tenth minute penalty corner goal scored by Army's Chandan Aind who later added two more goals to his and team's tally.
The star turn for the oilmen was provided by penalty corner hitter Sukhjeet Singh who also scored a hat-trick.
The other goals for the winning team were notched by seasoned campaigners and ex-internationals, Roshan Minz, Prabhjot Singh and Deepak Thakur.
Earlier, in an inconsequential Pool-A match, BPCL beat Indian Navy 1-0 with Gurpreet Singh scoring from a firm drag flick.
Both teams are out of contention for a semi-final spot.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)