Here's how I have been applying the charging rules in RMC:
The PC has a 50' movement rate and the target is 35' away reachable in a straight line. In order to have an attack, normal or charging, the PC must be running (2x movement); walking to the target would be a 70% activity (35'/50').
If the PC runs to the target and attack without charging, the attack would be at -35 because the movement is a 35% activity (35'/100').
If the PC charges in, he gets +35 to OB because the "final approach" is 35'. The charge effectively negates the penalty from the movement. The PC also gets a penalty of -35 to his DB, the same as the bonus to the OB.
In this example, the PC's decision is whether to go offensive (charge attack +0 OB, -35 DB) or defensive (normal attack -35 OB, normal DB).
The OB bonus differs from the movement penalty when either a) the movement required in the round is not all in a straight line to the target or b) the PC's base movement rate is different than 50.