=> First draft. Needs to be improved. Feel free to do that

Feel also free to ask questions in the comments they might be integrated here.

Frequently asked Questions:
aka. Why is my program not working.

Why can't I type anything in the kOS terminal? It's greyed-out and won't let me click on it.Edit

Question: Why is the terminal window always greyed-out and I can't click on it?
Answer: This happened when KSP 0.23 came out. The fix is to either use an unofficial build of kOS (see the thread in SQUAD's KSP forum website on kOS for several places people have posted such versions) or to wait for an upcoming Spaceport project that will continue kOS under a different author.
UPDATE: Such an update exists and is located

"set x to node(time + 10,0,0,0)" fails: 'is not a number'Edit

Question: Why does "set x to node(time + y,0,0,0)" fails?
I always get a 'is not a number' error.
Answer: Because the TIME object isn't a number but a bunch of numbers.
Solution: What you actually want to use is TIME:SECONDS.
Note: Don't mix up TIME:SECONDS which gives you the total time in seconds with TIME:SECOND which gives you the seconds of the current time. Which is always a value between 0 and 59.

Does kOS have functions?Edit

Question: Does kOS have functions?
begin surface_retro().
set result to velocity:surface + R(180,0,0).
return result.
set x to surface_retro().
Is there something like that, or is it at least planned?
Answer: No. But you can call subprograms with parameters which is doing roughly the same. Also until fixed (as in current behavior is not intended) all variables are global. that mean you can access every variable from every program and even outside of them.

I have a simple program but steering/throttle is not locking.Edit

Question: I have a simple program but steering/throttle is not locking.
It is looking like:
If I enter in the command line then it works.
Answer: If the program ends steering and throttle will be unlocked. That is a feature not a bug.
Solution: Try if adding WAIT 10. at the end does any change.
If it does then your program has ended prematurely. You can use several methods to prevent that.
Look at List of all Commands and KerboScript for further details.

Inputs meant for kOS are Interpreted by KSPEdit

Answer: Make sure that the kOS window is focused.
If that is not working you might have an incompatibility with an other mod.
Which is the fault of the other mod not of kOS!
However there are a few cases where some keypresses are passing through to both the KOS terminal and being seen by the KSP game as well and these are not all due to other mods. Examples of this are the X key and the del key. When X is typed into the terminal it will also kill the throttle in the game. This is a known bug and a solution hasn't been found yet. The problem is related to the fact that KSP itself handles the X key a bit differently than normal keyboard inputs, and it tends to bypass mods' attempts to lock it out.
See also Issues for known incompatibilities and further details.

Commands are not working not even on the command line.Edit

Answer: Have you ended it with a dot?

Program / Example XYZ is not working
or: I get errors but I am sure my code is correct.

Answer: There where quite a lot of changes in 0.8.x and bugs as well. The current version 0.85 is still very buggy.
Solution: Try instead an older version. I recommend 0.71.
See Issues for further details.

The kOS part is not showing up in Career ModeEdit

Answer: That wasn't implemented till 0.9.x .
See Issues or Modding Stock Parts for further details.
Solution: Please use the very recent version (of 0.9.2 or later). The kOS part appears in the same tech node as the first probe core. Beware that if you already unlocked that tech tree node before using kOS v0.9.2, KSP won't unlock the part until you go into the tech tree view, bring up the node, and click the kOS part (that wasn't there when you first unlocked the node).

My vessel spins wildly at first when I set the steering. Can I make it stop doing that?Edit

Answer: Not really. The KOS computer needs to "play" with the vessel for a bit to learn which way the controls are oriented and which way they work and this is what it's doing when you see it spinning wildly at first.
Follow-up Question: Okay, so can I at least detect when it has finished doing that so I don't have to execute a burn in the wrong direction, or have to guess what amount of time to wait before it's safe to burn?
Answer: Yes. While the property called STEERING gives you the direction you wish to be pointing, the property called FACING gives you the direction you actually are pointing. After setting steering, you can wait until the differences between STEERING:YAW and FACING:YAW, and STEERING:PITCH and FACING:PITCH, are both small to detect when the steering rotations have finished.