The bruise crew has pretty much already dissected this issue to death but I can still throw my 2 centavos in for consideration...
I am assuming a
pure wizard caster build in the following comments...
First, your wiz will reach no higher than level 18 by Ch 4 end so plan your 3 (max) ability bumps and 10 (max) feat selections accordingly, regardless of what your preference ends up being.
Expertise.
Actually, pretty useful in early going when you are trying to survive with less than 10 HP and before recipro shields are available. Not a necessary feat, but useful... and wizards get 3 bonus feats (per OC) to allow some diversification. Once all the primo DR and concealment buffs are available, standing AC level loses much of its importance especially when your wizard erects those nasty recipro shields... which brings me to an attribute build point...
CON (stitution).
A high-CON recipro wiz WANTS to get hit (except by archers, of course). There are a few beasties with resist/immunity to fire and/or acid but none to magic (ala Death Armor). So you can easily kill by just standing there sucking down full heals or using lots of kits. And there is no DevCrit in the OC, but there are some very high damage foes (like that scyther barb in Ch 1 that can hurt bad with a crit). High CON also increases Fort saves, typically the bane of many caster types and influences Concentration checks via the modifier. For the reason of high CON preference (and a few other race advantages except for the lost feat from using non-human), gnome is a natural choice. For a melee mage, it would be limited in weapon size, though. Human would be my second choice for a caster.
Armor.
Forget shield and armor with wiz. Unless you are planning on playing this toon into epic levels for autostill, managing Still Spell levels is a royal PITA (IMHO) and some spells cannot be stilled anyway (though at the moment I cannot recall the exceptions). If you decide to limit your casting only to pre-buffs without armor and then wands and scrolls in combat, equipping armor could work, I suppose. Spellcasting would be seriously diluted with that strategy, however.
Encumberance.
Use module caches. Store everything you can't specifically use
at the moment in one of the many available storage containers provided (for example, I like the chest outside Eltoora's tower in Ch 1). Just be sure to collect and/or sell excess loot before leaving the chapter. But as far as determining the minimum STR demanded in the environment...
This decision comes down to either trying to optimize the build for power by keeping the STR at min (8, or 6 for gnome) and maximizing the primary ability (INT) or just deciding that convenience is more important than optimization and boost an intial ability level a bit at toon creation for handiness sake. Perfectly viable either way. I agree about exploiting the magic bags in the OC and I buy every one I can regardless of the weight reduction rating because gold is not an issue, at least not after Ch 1 is over. Then keep trading up for better ones as the story proceeds. Besides, a wiz in the OC will not be lugging around heavy armor and shields anyway. Spend the paltry fee req'd to transport back to the temple to unload the heavy loot if necessary. Believe me. For a wiz that spends gold mainly for potions, scrolls and wands, most loot will simply be redeemed at rthe nearest vendor. Also, take note (if you haven't already, that is) that each point increase in STR will increase carrying capacity and does not rely on the modifier. So those +1 STR gloves will work fine in a pinch and a Bull's Strength potion/scroll lasts 3 hours. By level 7 wiz, another trick is to polymorph into a troll or umber hulk temporarily to lug a load to an unload point. The bottom line is: encumberance can be managed, even when capacity is very low. Just beware of the dead magic zones which will neutralize any magic item property or spell boosts (not a factor in the OC, fortunately).
WIS (dom) & CHA (isma).
Collect items that bump these two abilities (like nymph cloak & periapt ammy) and equip them just before beginning any convo or applicable encounter. Once again, it's a matter of power build optimization vs. convenience.
Skills: Concentration (spell interruption & Fort saves) & Spellcraft (saves vs. spells). Xclass to Tumble if AC is a problem for you. Listen/Spot only if you multiclass to a class that gets it as a class skill. Xclass a few points into Search for trap detection, especially at low char levels. INT modifer will boost this value significantly. There should be plenty of skill points for a wiz.
Feats: In the OC, Spell Penetration seems more helpful and influential than a single Spell Focus but in later (epic) levels, it loses its value quickly. Don't bother with any crafting feats, like Craft Wand, they can't be used in the plague campaign. Typically, Extend Spell, Empower Spell (or Maximize Spell), Toughness are selected. If you dede to melee and/or spennd a lot of time in within melee range, you'll want Blind Fight shoved in there somewhere. Skill Focus: Concentration wouldn't hurt either, if you can spare a feat.
So my preference for a starting wiz for the OC would be a gnome with INT of 18, CON of 16 and place the remaining starting points however you like. My surplus usually goes into Dex for reflex saves vs. traps & spells. All bumps to INT, in the OC, you will get 4 of them, (max). It really doesn't matter all that much. Not in the cakewalk OC, anyway. Just enjoy the learning, adventure and innate challenge and begin to improve your gaming technique for more difficult environments elsewhere.
All scalpels are welcome.
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