It's not likely that they are showing signs of pairing..
I would put my money on a dominance thing.. I have 4 O's in 2 tanks. The bigger set I rescued and they had lived together their entire lives. They were about 8 and 10 inches when I got them. They were fine towards each other. And then one day the smaller one turned on the bigger one! I am talking a MAJOR ass kicking that lasted a couple weeks.
I let them be and they sorted it... The bigger one healed, and they are still together today.
Only you can know when enough is enough... If you see severe damage, or major aggression, then you will have to decide when to separate...
Here is a pic of the damage to my now 14 inch baby!