rElec is important too for the Electric Golems, Storm Dragons, and Golden Dragons (I think, been awhile since I paid attention to what they were spitting at me).
These only spit fire, cold and poison attacks.
To the person who said you need rPois: no, poison is utterly irrelevant in Zot, as in just totally disregard it and carry a couple curing potions to cure it or confusion
I have done Zot with absolutely no rF or rElec and it sucked (one near-death experience). So I think rF+(+), rElec are "requirements" to do Zot reliably, and anything else is optional and potentially helpful. If you have low MR, watch out around aliches though and get them killed first.
10ish traps&doors is helpful in Zot - you don't want to be suddenly teleported or paralysed from traps!
"casters" (primary spellcasters, I take it) will all benefit from the following:
1) a solid melee attack to save MP, hunger and noise (mainly the former)
2) Charms skill and Swiftness, Control Teleport, Repel Missiles, Haste, Regeneration (ridiculous spells for low costs: 12 Charms)
3) Translocations skill and Blink, Control Teleport, Phase Shift, Apportation, Controlled Blink (ridiculous spells for medium costs: 14 Translocations)
4) a form of replenishing MP: Sublimation of Blood, staff of channeling/Wucad Mu, crystal ball of energy, Sif Muna, Vehumet
5) Fighting skill
6) Dodging/Armour / Dodging+Armour depending on armour
7) mottled dragon armour (unless you are low on rF or find some ridiculous randart)
re: Chei, Bend Time works against really high-MR dudes as well... (not orbs though)
re: Oka, no reason to save piety so just spam Finesse/Heroism
also, just use all of your consumables, save some tele and digging for the orb run