Already locked in as the No. 2 seed for the Summit League tournament, the Golden Eagles (17-14, 13-5) had little trouble dispatching the ninth-place Leathernecks (7-22, 2-16) for their eighth straight win.
Western Illinois kept the Eagles in check early with its brand of slow-paced, methodical basketball. But a 14-2 run late in the first half gave Oral Roberts a 29-19 halftime lead that it wouldn't relinquish.
Roundtree had 19 points and 13 rebounds, while Bell-Holter finished with 13 points and 11 rebounds.
The Golden Eagles will play either North Dakota State or Southern Utah in the first round of the Summit League tournament March 5.