You do not need opening points to bid after an opponent has opened the bidding. There is only one opening bid and it has been made by an opponent. You are now the overcalling side and can bid with many fewer points.

There are many advantages to entering the bidding with a good suit even with a low point-count:

(i) You disrupt the bidding for the opponents, making it harder for them to reach their optimum contract.

(ii) You may be able to outbid them to a making contract.

(iii) Perhaps more likely, you may be able to outbid them to a contract that will fail, but concede fewer points than the value of their making contract (a ‘sacrifice’).

(iv) Even if the opponents end up declaring, you indicate a lead and defence to partner.

With so many plusses to entering the bidding, you should do so frequently, with the proviso that because you may be left in your suit (possibly doubled), it is vital your suit quality be good, even if your point-count is not (six/seven or so points in the hand is sufficient for a one-level overcall, eight /nine or so for a two-level overcall).

Use the Suit Quality Overcall Test (‘SQOT’):

bid up to the trick level that equals the no. of cards in your suit (min. five) added to the no. of honours in your suit (count one for A, K, Q, J, 10).

Bid 1 ♠ over 1 ♣ with these suits:
♠ KQ852, ♠ A98532, ♠ Q10975.

Bid 2 ♥ over 1 NT with these suits:
♥ AJ1085, ♥ KJ9742, ♥ KQ1093.

Note that you would bid 1 ♥ over 1 ♣ with each – overcalling at the lowest level even with a Suit Quality Count that permitted you to bid higher (if needed).