I would say the best offhand for range in Old School RuneScape is a bow. You can find them easily on the ground floor of Varrock Palace, or buy one from any general store.
The main reason you want an off-hand bow is because it will reduce your inventory space requirements by 1/3rd (rounded up). So instead of needing 6 inventory slots, only 3 are needed! And since bows require no ammo, they are also much less expensive to maintain than ranged weapons that use arrows/bolts.