If armed, roll weapon damage +6
This matches the description of the other ogre varieties e.g.:
1-10 (or by weapon +6)
As well as the section describing the monster attributes at the start of the book:
If the monster uses weapons, the damage done by the typical weapon will be allowed by the parenthetical note "weapon." Damage bonuses due to Strength are listed as a bonus following the damage range.
The equivalent stat block style description for the chieftain would be: "2d6+6 (or by weapon +6)"
Ogres are large and can use large weapons one handed as per the player's handbook:
A character can always wield a weapon equal to his own size or less. Normally this requires only one hand..
They could use giant weapons with two hands if they could attain them.
A character can also use a weapon once size greater than himself although it must be gripped with two hands.
Taking a cue from the fire giant's description, a giant weapon would do double damage to medium or smaller creatures.
A fire giant’s oversized weapons do double normal (man-sized) damage to all opponents, plus the giant’s strength bonus