I have a script that grab the objectid of currently equipped arrows, unequip it, and then re-equip it, for personal purpose. The arrows are already in my inventory

the problem being the re-equip part equips only 1 out of 500 arrows I currently own of that type, splitting it into 1 (equipped) and 499 (in inventory).

Is that the way EquipItem is supposed to work?

Yes, it is working correctly. Not ideally, but correctly.

Luckily, there is a workaround. Before equipping the arrow, remove all but one from the inventory. After equipping the arrow, add the arrows back to the inventory. Oblivion auto-equips the added arrows if one of them is already equipped. If I recall though, this will leave your quiver looking like it only has one arrow in it. Not really a problem unless you play in 3rd person :shrug:

I just doomed myself: I primarily play in 3rd person. Heck, after I re-installed Oblivion, unless to focus my crosshair precisely on something I never use 1st person, especially after I installed a mod that allows me to see the 1st person crosshair icons in 3rd person

Maybe I'll leave the arrows alone =/ damn arrows....