I just started with a medium sized company in Ottawa, and I noticed a lot of engineers have one-handed pocket knives. ie: The kind you can push open with one thumb. I'm not comfortable working in an office where so many people carry weapons. Assuming no laws or company policies are being broken, how should I approach this?
Well, I'm going to have to first address the loaded language in your post. Many people carry pocket knives solely or primarily as tools. In the case of most of Canada, save for in the wilderness, a pocketknife MUST be solely used as a tool, no matter the circumstances, or it is categorically a weapon. You seem to be conditioned into thinking knives are strictly weapons, when they are typically used as tools. In most legal jurisdictions, a tool like a pocket knife is not a weapon until it is used as a weapon, or the owner states that it is held with the intent for use as a weapon or as a weapon of opportunity.
Second, how large a knife are we talking? There are no length restrictions in most of the country, and so long as it's not a flick blade, push knife, switch blade, or balisong/butterfly-knife, it's likely legal. I am not a lawyer (IANAL), so when in doubt, consult a lawyer on this topic. Also, you would do well to learn your local knife laws:
It sounds like your coworkers are using either thumb-loop or thumb-stud assisted openers. Most variations of these are indeed legal. Are they waving them around or duelling with them or trowing them at walls/dart-boards? That could be grounds for HR getting involved, but not if they're just opening boxes, cutting fruit, etc with them.
Thumb loop one handed opener
Large lockback models are also fairly common:
Personally, I try to avoid anything larger than 4" since that seems to be the unofficial "rule of thumb" in my locale, at least as far as the police are concerned. Also, when travelling to the US, there are states with city-specific laws. I think Seattle is a 2.9" limit, while neighbouring areas it's 3.5" or 3.4". Kinda funny there are places where you can walk around with a concealed handgun, but a tiny pocketknife is a criminal offense.
Update: Every single answer so far points out that the problem lies with you and your being conditioned to fear tools. Your best advice across the board is to either find new work, or work on de-conditioning yourself, not necessarily in that order.
If your co-workers wished you harm, they could just as easily do so with other 'innocuous' items you're conditioned to overlook (pencil, pen, scissors) while you've been conditioned to fear knives which are admittedly just as dangerous but also just as useful (if not more so) as all the other items.
I work in an IT firm as a Biz-Alyst/Proj-Mgr where I rarely need to get my hands dirty. Still, I often carry a Leatherman C33T tool (2.6' blade) to the office. It works great as a spare utensil, box cutter, envelope opener among other uses. Some of the knee-jerk reactions I initially received were much like yours -- usually from folks that spend too much time on the couch and not enough outdoors. My knife has been a common sight for so long that my co-workers no longer react and now come to me when they need one -- for legal purpose of course.
Canada's knife legislation is more than reasonable and I'd assert that whether or not you choose to carry a knife, that you respect the rights of those that do provided that we do so: (a) For use as a tool (b) Not in a public meeting (c) Not for criminal purpose (d) Not for purpose dangerous to public peace
Reasonableness comes into play here. A pair of scissors can penetrate just as deeply and be just as deadly as a knife. A knife in the hands of a law-abiding, sane, and emotionally mature co-worker should be no different in your mind than a pen or pair of scissors.
Length also plays a role. I could certainly empathize with my co-workers feeling intimidated and fearful if I were to have a Ka-Bar or SCHF9 strapped to my hip around the office. In their mind (and even mine), why would I need such a large tool and what if I a deranged co-worker were to avail themselves of such a hefty blade? Even if I had a valid reason (perhaps a friend will be picking me up after work to go camping and I needed to bring my equip to the office), in my opinion it's a bit much. Granted my co-workers would merely look at me with little more than slightly raised eyebrows but I'd rather not set a precedent and would just leave the survival knife in my backpack and use my smaller knife as my office tool.
As for your co-workers: life is never 100% certain, safe, or secure; but unless they're carrying around over-sided blades without reason, it's possible that you may be (over-)reacting out of conditioned fear.
De-condition yourself ;)
The same object may be a tool or a weapon depending on context. For example, a very sharp knife with a point would be a weapon at e.g. a rather rowdy sports event. It would be a tool, suitable for paring fruits and vegetables, if found in a working chef's knife kit.
The sort of knife you describe, carried by engineers on the job, is more likely to be a tool than a weapon. Think about what should happen if, for example, a cable tie needs to be cut. Do you really want the engineer to have to go to a tool crib to get a knife, cut the tie, and take the knife back again? That would waste a lot of time.
Your best approach is to recognize the engineers' knives as practical tools rather than weapons.
I'm not comfortable working in an office where so many people carry weapons.
I think in certain rural parts of the US you'll see it too. I'm sure in Texas somewhere, or out in Montana etc.. Not much you can do, since it's cultural.
Since it seems rather endemic to the work area, your best bet is to follow Joe's advice: look for a more comfortable work.
If someone you sit nearby every day carries it around, you could politely try asking them.