What's the opposite of "newcomer? Is "veteran" OK?

Someone who has just come to a new workplace may be called a "newcomer". What about someone who has been at a workplace for a while or a long time?

Does veteran mean this? I looked it up in the dictionary; Collins says "You use veteran to refer to someone who has been involved in a particular activity for a long time." But I do not quite think this word fits. Are there any other words for this? What about "old people", "old man", "old comer"? Are they OK?

If you only want to remark on their time spent in a place or doing an activity, then you could use informally old-timer, professional (figuratively, too) or grey-hair. Or, more formally, senior might work (but it does often suggest high status as well). If you don't mind suggesting that the person is also skilled, then there are a bunch: Expert, guru, authority, etc.

To answer your question, yes, veteran is acceptable, but a little cheesey (in my opinion).

Old-timer sounds very American to my ears. If you want to avoid the connations of veteran, another option would be "old hand".

From Cambridge:

Noun: Someone who is very experienced and skilled in a particular area of activity:

We should be able to trust Silva to negotiate a good deal for us — he's an old hand at the game.

There are many words you can use as an antonym of newcomer.

Efficient, adept, versed, dexterous, deft, masterful, old-timer etc..

Veteran is technically correct, but it is uncommon to use the word veteran as veteran is usually (but not always) used to refer to a(n old) person who was on the war field, ie. a retired soldier. The opposite of new comer would be old timer. Calling someone who's already been at the workplace longer than you have a senior is also fine, as you're their junior.

Edit: As taken on my response to Janus' and Dodgie's comments:

Good point, as a matter of fact senior does commonly imply age. I just went ahead and assumed that someone newer would be younger while in fact it's not necessarily true. I'll put this on the main answer.

The word senior would be common to use when the old-timer is older than you.

Note that in some countries it is not uncommon to use senior to refer to someone who's been at a workplace for longer than you have, this depends on the culture of the countries the workplace is located in.