Suppose I had a hand like the following: (s) KJxx (h) xxx (d) AJx (c) AKx. If I were the opening bidder, I would, of course, bid 1NT. (Note to commenter below: In this paragraph, I was describing a "generic" balanced 4-3-3-3 16 hcp "no trump" hand with honors in three suits. This was NOT a specific question about the particular hand configuration above.)
If someone opened in front of me with 1 club, diamond, or spade (all of which I have a stopper in), but not 1 heart, I would still bid 1NT as an overcall.
The earliest I could do this is in the second seat, after the original bid. In this case, I know little about the holdings of either the opponent's partner or mine, except that they are constrained by the strength of the first two bidders.
If I overcalled in the third seat, I would know that my partner (first seat) was a passed hand but I wouldn't know this about my opponent's partner (fourth seat).
If I overcalled in the fourth seat, I would know that both my and the opener's partners had passed hands.
If the "one bid" came from the first seat instead of the third seat, I would be in the "balancing" seat, and possibly overcall 1NT with 12-14 points.
Do I need to be more careful in the third seat, since I might be sandwiched between two strong opponents?
If the "one bid" came from the third seat after two passes, can I overcall light in the fourth seat as if I am "balancing," even though technically I'm in the direct seat? Expert Larry Cohen calls this "balancing in the direct seat" and advocates this.
If opener bid a suit in which I have stoppers (that is, other than hearts), can I worry "less" about he heart suit, on the theory that since opener didn't show strength in hearts, my partner and the opener's partner are equally likely to have strength in that suit?