CRO is a system for bidding 2 suited hands. These are immediate bids over an Opposition 1 level bid!
There are 3 bids, in the main:

  1. a cue bid (C for Colour)
  2. a 2NT bid (for the same Rank)
  3. a 3 Club bid, (for Other, or Odd if that is easier to remember)

The Cue Bid (Colour) will always be a one level raise of the opening (opposition) suit, i.e.

Oppo You      
1♣ 2♣   Shows the red suits, diamonds and hearts ♦, ♥
1♦ 2♦   Shows the black suits, clubs and spades ♣, ♠
1♥ 2♥   Shows the black suits, clubs and spades ♣, ♠
1♠ 2♠   Shows the red suits, diamonds and hearts ♦, ♥


The Ranking suits will always be the minors(rank) or the majors(rank)

Oppo You      
1♣ 2NT   Shows the majors, hearts and spades ♥, ♠
1♦ 2NT   Shows the majors, hearts and spades ♥, ♠
1♥ 2NT   Shows the minors, diamonds and clubs ♦, ♣
1♠ 2NT   Shows the minors, diamonds and clubs ♦, ♣


The Other(Odd) suits will always be a major, and a minor of the other colour

Oppo You      
1♣ 3♣   Shows the other minor, diamonds, and the major of the same colour, spades ♦, ♠
1♦ 3♣   Shows the other minor, clubs, and the major of the same colour, hearts ♣,♥
1♥ 3♣   Shows the other major,spades, and the minor of the same colour, diamonds ♠, ♦
1♠ 3♣   Shows the other major,hearts, and the minor of the same colour, clubs ♥, ♣


Lets look at a few bidding sequences, which do NOT apply to CRO.

Oppo Us Oppo Us    
1♣ P P 2NT   This is NOT an immediate bid of 2NT, therefore should be 17-19 points
           
*2♥ 2NT       (* Weak 2 bid) Again, this is showing 16/17 - 19 points, balanced, good hand (heart stop)
           
*2♥ P P 2NT   (* Weak 2 bid) Once again, a balanced hand of 16/17-19 points, with a heart stop or 2 (Ideally)

Some examples. Your right hand opponent opens 1♣ what should you bid with the following hands?

(a) ♠ 7 6   Just overcall 1 heart (any vulnerability) Cannot do a jump overcall of 2H (no 6 card suit); happy with a heart lead, if opponents win contract.
  ♥ K Q J 7 4  
  ♦ 6  
  ♣ A 10 8 4 2  
           
(b) ♠ K Q 10 8 2   Overcall 2NT (Any vulnerability) RANK 6 losers, 5/5, perfect bid
  ♥ Q 8 7 4 2  
  ♦ 7 2  
  ♣ 5  
           
(c) ♠ 8 7 6 4 2   Pass! Far too much in defence you will make 3 tricks probably. You have no honours in the 2 suits.
  ♥ 8 5 4 3 2  
  ♦ A K  
  ♣ A  
           
(d) ♠ 5   2♥ (Weak jump overcall) Don't worry about having 5 in their suit. There are a lot of spades out - the opposition may have them,
and not be able to bid them
  ♥ K Q J 7 5 2  
  ♦ 5  
  ♣ Q J 10 7 4  
           
(e) ♠ 10 9 7 5 3 2   Overcall 2NT Perfect with 11 cards, Don't worry about spades being weak, if Partner has ♠ Q J and ♥ 7 2 he will bid hearts; You would prefer to be in 3 Spades
  ♥ A K Q 7 2  
  ♦ 6 2  
  ♣ ~  
           
(f) ♠ 10 7 5 4 2   Overcall 2NT any Vulnerability  
  ♥ K Q 10 8 7  
  ♦ K Q 8  
  ♣ ~  
           
(g) ♠ K Q J 8 7 2   overcall 2♠ (or 3♣) with 11 cards, you could bid 3♣ (Odd suits - Spades and diamonds), but 2♠ (Weak Jump Overcall) is better.
  ♥ 7  
  ♦ 10 8 5 4 3  
  ♣ 6  
           
(h) ♠ 5   Overcall 3♦! Overcall 3♦! (Or 4♦). Where are the major suits? Make things difficult
  ♥ 8  
  ♦ K Q J 10 7 4  
  ♣ Q 9 8 4 2  
           
(i) ♠ K 7 5 3 2   Only bid 2NT if not Vulnerable If Vulnerable, too much danger of giving a large penalty
  ♥ Q 8 5 4 3  
  ♦ 8 2  
  ♣ 5  
           
(j) ♠ A K Q 10 8 4   Bid either 3♣ or double On this game going hand (you can count 2 losers only), there is a slight problem, if you bid 3♣, that Partner
will pass ( i.e. ♠ 5, ♥ 8 5 2, ♦ 7 2, ♣ Q J 10 8 7 4 2). Probably better to double.
  ♥ 5  
  ♦ A K 10 7 5  
  ♣ A