After Opponent's Opening bid, a 1NT Overcall traditionally shows a balanced hand with a stopper in the Enemy suit and a strength (depending on the system used) within the 15 to 18 points range.
Besides the fact that such Overcalls are rare, they also are rather superfluous.

Why not double first for take-out, then bid NT at your next turn ?
By using this second option, the 1NT Overcall can be used for other purposes.

Very Unusual 1NT Overcall after Opponent's 1 in a suit Opening bid

(1♣) or (1♦) → 1NT = 5-8 HCP, 7 losers, 5+5+ in the 2 major suits

(1♥) or (1♠) → 1NT = 5-8 HCP, 7 losers, 5+5+ in the 2 minor suits

(1♦) - Pass - (1♠) → 1NT = 5-8 HCP, 7 losers, 5+5+ in the 2 unbid suits
      Minimum Suit quality in the long suits : H x x x x   H x x x x     (H = Honour card)

The "Advancer" (Overcaller's Partner) has the following options :

1NT → bid preferred suit at cheapest level (8+ losers, 0-12 HCP)

1NT → bid Game in preferred suit (6-7 losers, 13+ HCP)

1NT → bid Enemy suit = Strong enquiry (7 losers or less, 13+ HCP)
            (or DBL RH Opponent after Enemy interference)

• next step up the line = minimum, 5-6 HCP
• 2nd step up the line = maximum, 7-8 HCP