Two things to think about:
In computing, it's common to use failure instead of just fail, because it happens to be the same number of characters as 'success', so in mono-spaced fonts they end up being the same width. (Side note: "warning" is also the same number of characters) In addition to this, some systems (like Linux/Unix) color the words - failure is usually colored red, and success is usually colored green/blue. So in these systems it's not the [dis]similarity of the words, or the fact they have the same character width that helps identify warnings/errors, it's the color that helps identify failures.
Word coloring is one technique I've seen in the aerospace industry, used with actual avionics equipment in cock pits.
The fact that success, failure, and warning are the same length means that the log message will start at the same location on each line. Eg:
timestamp | action | FAILURE | message/description
timestamp | action | WARNING | message/description
timestamp | action | SUCCESS | message/description
Which can help many people with readability. It's easy to ignore the first 3 columns if they're all the same width. And once you've identified failures/warnings, the most important columns for a human is probably the message/description. Action may be important, but if you see the error message you'll usually have a clue about what to do regardless of what action was taking place. Consider this [slightly contrived] counter-example where each status is a different width:
timestamp | action | fail | message/description
timestamp | action | successful | message/description
timestamp | action | warning | message/description
Now think of seeing that with dozens or hundreds of lines - it gets hard figure out where you need to focus your attention. Of course the simple fix there is to pad the status field with spaces, but then you might lull yourself into discerning status based on word width in the status column (which seems to be your current dilemma).
At least in the aerospace industry, I've seen the use of "GO/NO-GO" for indicating status. NO-GO can also be represented as NOGO or NGO, which is similar to your NOK. I would argue that this is much more widely known, used, and easy to understand for newcomers. And it assuages the concerns many commenters and some answers have pointed out in the more uncommon "NOT OK"/"NOK".
So I would probably do a bit of everything, if possible - take Andrew Martin's answer and use a symbol, color the letters, and pad spaces so the actual messages that you (as a human) want to read are all lined up. Of course this all depends on what kind of UI/format we're talking about - terminal? text file? rich text file? HTML? etc