It depends on the system.
In my job, I wear many hats, one of which is Operating System Administrator for the core business ERP.
By "hat" I mean playiing roles of programmer, analyst, computer security, data base accounting forensics, etc. in which each calls for a different mix of responsibilities and expertise.
For the hat of OS administrator I would guess the most important skill is knowing how to look up facts needed, where to go to get answers. This is because an OS is enormously complex with an infinity of facets, many of which we never need to have personal involvement with. But then something goes wrong, haywire, appears to be malfunctioning, and we have to react extremely rapidly.
For example ... let's suppose there is a runaway job, generating some kind of log or report that is never ending. This begins to eat disk space, CPU & Memory resources. We, the OS administrator, or other system trouble shooter, need to detect that this is happening long before the end users notice that something is wrong. We need to figure out what job is doing this, and bring it down in such a way as to capture essential information so that the job can be repaired, so it won't do it again. If a runaway job is not intercepted fast, like within a few hours, you could end up without a system, without the capability of making repairs. Where I work, we do have runaway jobs occasionally, and I do bring them down within minutes of detecting them.
This is an example of something, where not only do you have to know where to find answers, you need to be able to find them really fast.
For my OS administrator job, I guess the second most important skill, or knowledge, is an awareness of the mixture of attributes that need to be managed, and their relative importance or trade-offs.
Performance is an important attribute. This means efficiency of the resources that the people and computer systems utilize to get the job done.
Let's suppose you go to some web site, or screen, or load some program, and you get a "please wait" message, while something is loading. That is poor performance.
Let's suppose you key in a bunch of transactions on some screen, then submit the data to the computer system to do its thing. Now with my programmer hat, I know the program may be execuring millions of lines of instructions, accessing scores of records in different files, communicating over hundreds of miles, but if the end user has to wait for the answer, where there is a noticeable few seconds before the reply, that is poor performance.
It is especially poor if someone has a bunch of transactions to key in ... fill the screen, submit the data, wait 2 minutes, fill screen again with more data, submit, wait 2 minutes. That goes on all day. The company could get significantly more value out of that employee without all those wait 2 minutes for the data to be processed by the computer system.
That performance problem can be tackled many ways.
Change the program
Get faster communication line
Get more memory
Get faster hardware
But it is the OS administrator's job to understand how the different pieces of the computer system play a role in performance and how to measure what will make a difference.
In my case when there was a management complaint about performance where they specifically asked how to get the best bang for the buck in fixing it, my analysis came back saying that what we needed was more memory for cache, since we were getting seven to one hits on cache efficiency (meaning 7 times the data needed was in memory cache, and 1 time the needed data had to be extracted from hard disk), but only 10?f the time the job was waiting on something due to a clog in communications capacity.
As a result of the company investing $ 2,000 in additional system memory, the work force all noticed a significant increase in system performance.
This was a case of me as OS administrator knowing how to get the answers to questions, how to measure performance to figure out where the bottlencks were that could be fixed, to get best bang for the corporate buck.
OS Performance is only one of many areas of responsibility for the OS administrator, but for each one, in my opinion, the most importand skill needed is knowing how to get at relevant information, rapidly, and to be able to act on it.
I have no idea what b.c.a means
so obviously b.c.a is totally irrelevant to my job as an OS administrator
Incidentally the OS that I administer is OS400 from IBM ... now some people may say "Hey Al ... IBM has improved OS400 ... it now has a new name" which is true, but I administer an old and reliable system that is in fact running on OS400. That is the correct name for the OS that I am administering.
For additional insight you might check out sites like http://www.interviewrx.com/index