RMU says:
1. Animate Dead I – Caster can cause any dead body (within 50% of his own mass), to rise up and move. The caster must concentrate for the body to perform an action; if he stops concentrating, the body will stand motionless. The body can be maneuvered with a -25 modifier and can melee with a -20 OB. The body cannot have been dead more than 1 day. Use the stats from Creature Law for Class I Standard Created Undead except that the OB is -20 and it cannot move faster than a walk.
So that's not creating undead, like you say, it's just puppeting a dead body. The OB is going to be the listed OB of a Class I standard created undead except reduced by 20.
The RMSS version is the same except it points you at Creatures & Monsters for the stats. As noted above, class I standard created undead in RMSS have an OB of +25 small bash or +15 weapon attack. So if you are using the 4th level Animate Dead II which only reduces the OB by 10, then the dead animated by this spell will be making their attack as a +15 small bash (25-10=15) or +5 weapon (15-10=5).