Biswal struck an unbeaten 91 (103m, 90b, 7x4, 1x6) and was involved in a 144 run partnership for the second wicket with Das (90; 172m, 138b, 15x4) as Orissa reached their target of 219 for the loss of only two wickets with 27 balls to spare at the Eden Gardens.
Biswal and Sanjay Raul (29 not out) then stitched together an unbroken 75 run stand for the third wicket to ensure four full points for their side, who now have 13 pointsin their kitty from three outings and are already assured of a place in the final round.
Earlier, Assam recovered from a top order collapse to post a decent 218/7 in 50 overs after being put into bat by Orissa skipper D S Mohanty.
For Assam, S Ghosh top scored with an unbeaten 59 (103m, 79b, 2x4, 2x6) and put on 85 runs for the seventh wicket with S Singh (49; 69m, 65b, 5x4, 1x6) to retrieve the situation after their side were precariously placed at 105/6.
Brief score : Assam 218/7 (S Ghosh 59 no, S Singh 49; D Mohanty 2/31, S Satpathy 2/39). Orissa : 219/2 (S S Das 91, S Biswal 90 no).