The pair could make their debuts against Bedford Town on Saturday. The transfers are subject to league and FA ratification.
Attacker Hallam (24) has made a total of 33 appearances in all competitions for the Iron since signing in January 2019, scoring three goals, and has recently made a return from long-term injury.
Left-back O'Malley (21) has played 79 times for United, including 20 times in the National League this season, and his most recent appearance was against Bromley a fortnight ago.
