The opening credits for Jujutsu Kaisen generally only feature scenes of the major characters who will appear in the forthcoming episodes. As far as I can tell, there is only one motif that actually recurs between the two OPs in season 1, which is that of a thin black and white fish.
Screenshots from the first OP:
Screenshot from the second OP:
What does this fish symbolize?
I tried searching for images of black and white fish, but I wasn't able to find any real fish that closely resembled the one pictured in these screenshots.
It's possible that the black and white pigmentation indicates that the fish is poisonous; if it is so, then my guess is that it's supposed to represent Ryomen Sukuna, since he's also known as the King of Deadly Poisons. I also get the vague feeling that it could have something to do with Yoshino Junpei, but I am unable to articulate why this would be so. Unfortunately, there's not much that I'm able to say beyond this.