O/S Upgrades
Do you feel a little bit limited by the existing Upgrades available in System Shock 2? Are you hoping to specialise your character just a bit more? That's where the O/S Upgrades come into play. O/S Upgrades are one-time, passive improvements to your cybernetic rig, which give you some sort of bonus, discount, or additional ability.

There are four O/S Upgrade Units scattered around the Von Braun and Rickenbacker (MedSci Medical, Hydroponics B/C, Recreation C, and Rickenbacker Bridge). You'll encounter two of them fairly early on, a third midway through the game, and the last just before the end, so it's best to take the two you feel are most important first, especially if the bonuses it provides become more important later in the game. Note that there is no confirmation button when selecting your upgrades - click only when you're absolutely sure what you want.

Though there are 16 O/S Upgrades in total, not all are made equal. The unfortunate fact is that while many might sound good on paper, in practice there's only a handful of them worth getting, though some are useful in a specialised context depending on your character. Considering the limited number you'll have throughout the game, you'll want to pick and choose from the best depending on your play-style.

Antipersonnel Pistol Rounds Strong Metabolism
Description: Damage from radiation and toxins reduced by 25%.
Assessment: Radiation and toxins are minor threats throughout the game, and there are plenty of ways to deal with them (Hypos, Hazard Suit, etc.). Not very useful compared to many others.

Antipersonnel Pistol Rounds Pharmo-Friendly
Description: Extra 20% benefit from all Hypos.
Assessment: One of the better choices. Hypos are your main source of healing and essential to Psi users, so this will, in theory, reduce the number you need to use.

Antipersonnel Pistol Rounds Pack-Rat
Description: Adds three extra inventory slots.
Assessment: Useful early on, but only get this if you don't plan on increasing your Strength at all. Midway through the game you'll probably have all the inventory space you need.

Antipersonnel Pistol Rounds Speedy
Description: Movement speed increased by 15%.
Assessment: Like Pack-Rat, this provides an effect similar to increasing an attribute without providing all the benefits. Can be useful early on, but you're better off just increasing your Agility a level or two.

Antipersonnel Pistol Rounds Sharpshooter
Description: Ranged (non-Psi) weapons do 15% more damage.
Assessment: One of the most useful O/S Upgrades unless you focus on melee or Psi. Ammo is your most important resource and this will make each shot count a little more. Applies to Heavy and Exotic weapons too, so pick it up even if you don't plan on using the Standard weapons.

Antipersonnel Pistol Rounds Naturally Able
Description: One-time bonus of 8 Cyber Modules.
Assessment: This will seem tempting early, but unless you avoid all exploration, you'll have more Cyber Modules than you know what to do with later in the game. Only get this if you feel the early-game benefits of more Cyber Modules outweigh all other options.

Antipersonnel Pistol Rounds Cybernetically Enhanced
Description: Allows the use of two implants at once.
Assessment: One of the best choices, hands-down. Implants provide better bonuses than many O/S Upgrades themselves, and while they require recharging, the versatility offered by them makes up for the drawbacks.

Antipersonnel Pistol Rounds Tank
Description: Increases maximum Hit Points by 5.
Assessment: Near-useless, even when playing on Impossible difficulty. If you need more health, 5 Hit Points aren't going to help much - you're better off boosting your Endurance instead.

Antipersonnel Pistol Rounds Lethal Weapon
Description: Increases all hand-to-hand damage by 35%
Assessment: Essential for melee fighters, and even worth considering if you plan on using ranged weapons, as the Wrench will be your most-used weapon early on.

Antipersonnel Pistol Rounds Security Expert
Description: Provides a +2 Hacking skill bonus for Security Control Stations only.
Assessment: Near-useless. You'll want to improve your Hacking skill throughout the game anyway, and most Security Control Stations aren't even that tough to hack.

Antipersonnel Pistol Rounds Smasher
Description: You can execute overhand power attacks with melee weapons.
Assessment: Only useful for melee-based characters. Combined with Lethal Weapon, it will make your melee attacks extremely powerful.

Antipersonnel Pistol Rounds Cyber Assimilation
Description: You can extract Repair Modules from destroyed robots, which heal 15 Hit Points when consumed.
Assessment: Mediocre ability. There are enough sources of healing to make this redundant, and robot enemies are less common than organic ones; only consider this if you are having a huge amount of trouble staying alive.

Antipersonnel Pistol Rounds Replicator Expert
Description: All items bought from Replicators cost 20% less.
Assessment: One of the better upgrades, regardless of character. Replictators will be one of your best sources for certain items throughout the game, and when also hacked, you can get some very big discounts.

Antipersonnel Pistol Rounds Power Psi
Description: Psionic burnout (charging a Psi ability too long) no longer damages you.
Assessment: Not very useful. Burnout does not do much damage, except for the highest-tier abilities. Only worth considering for 100% Psi-only characters.

Antipersonnel Pistol Rounds Tinker
Description: Nanite cost for modifying weapons reduced by 50%.
Assessment: Not very useful. You'll likely only modify a few weapons during the game, and you'll have more than enough Nanites for that.

Antipersonnel Pistol Rounds Spatially Aware
Description: Automap for each level is automatically revealed.
Assessment: Mediocre overall. At best, this will show you where Upgrade Units and similar are, but you'll probably be exploring everywhere, anyway. Doesn't work on the final levels of the game, either, where there are no maps regardless of this ability.