I had a debate about this subject on the topic of Eng claymores. As far as grenades I do not believe they should be OHK because they can be spammed. Sure there is a cooldown in place but that does not stop a team from throwing a bunch in which case you would die anyways because the team worked together.Also you have to take into account they can be cooked so no one has time to react. Also they do knockdown which is great considering you could throw a grenade into the middle of a group of enemies knocking them all down leaving them vulnerable to be taken out by your team.
I do believe however that claymores, sticky bombs, and satchel charges (not really ohk but with mutliple ones) should be OHK.
Claymores- These are a class specific ability meaning there will not be some crazy amount all over the battlefield. On top of this they can only lay down 1 and there are mechanics in place to keep them from being OP. First off they can be seen like neon signs by Operatives, on top of this the operative can spot and mark them making there position revealed on the mini map for all teammates. Then there is the disarm ability meaning that if you step on one as long as you do not step off it can be defused by an Eng on your team. Add to the the risk setting one up and the fact that someone stepping on it is a pure chances game and I see no reason why they should not be OHK. If someone steps on one and is killed with all this in place then they surely deserved to die.
Stick Bombs- I believe should be OHK because again with the mechanics involved with them it makes sense also another class specific ability. The sticky bomb has to be stuck to an enemy for it to be effective. Not a small order in Brink with the SMART system in place and all the crazy movements that will be going on. The most effective way this will prob be used is by disguised Operatives who throw them on unsuspecting enemies. With that said even after they get it on them the victim has an allotted time period in which to get to a teammate who can defuse it for him. With that said again I see no reason it should not be a OHK.
Satchel Charges- Another class specific ability. These are suppose to be quite large meaning they will more then likely be easy to spot. But a truly clever soldier will plant these where the enemy cannot see. They can be stuck to any flat surface. These are remote detonated and 3 can be thrown. They can be individually detonated or all at once. With that said I am sure Operatives will be able to see these like neon signs as well spot and marking them for there team.On top of this they can be shot and detonated by the opposing team defusing the situation. Here is my suggestions on how OHK should work for these. 1satchel charge is enough to take out a light,2 a medium and all 3 a heavy. I think that would be fair mechanics and with everything in place see no reason why they shouldn't be OHK.
As far as certain weapons being OHK I do not agree with. Sure they may not do OHK if you shoot someone with a shotgun in the head but it is going to severely injure them compared to if you shot them in the chest. There not being OHK for guns at first I was skeptical about but think about it like this all those people that spray and pray and get a lucky headshot... well hey that wont be happening anymore if you are truly a better shooter then you will more then likely win that gunfight because you will get more crit hits landed then them.
On top of this when I am mentioning a OHK above I just mean to incapacitate not completely take them out because no matter what you have to go to that state before you can die in Brink. I think when you add this on top of eveything there should be no reason why these are not OHK.
Mine < Sticky nade < Satchel charge damage wise
I believe mines should OHK a light. (Standard engineer kit)
Sticky nades OHK a medium (Rank 2 Operative)
Satchel hard OHK a heavy. (Rank 5 Soldier)
Those should be unbuffed and no perks. This would make sense based on the rank and relative strength of the abilities.