Possible issues.

I have a huntsman regal XL non regulated. First and only pcp. 

When I first got the gun about 8 months ago I accidentally fired the gun with the action open. Three time. I didn’t see any damage to the o rings but I ordered extra anyway. I have yet to replace it and the gun seems to shoot ok. I can hit stuff with it have made some long shots on small game. But at 20 yards I have a good bit of flyers. Sometimes pellet on pellet other times 1 inch groups. Should I try and replace it? Or wait till I can see damage? I’m new to the sport so I know my issues could be from many other things. 
 
You might try the tissue paper test-put a piece of tissue paper over the breech/magazine area and take a shot with the bolt closed. See if the tissue gets blown off, if it does you have an issue at the breech seal.

As far as your groups are you shooting off bags? Have you tried different pellets? (H&N and JSB usually do well) Do you have a level on the breech or scope to minimize cant? Have you made sure your scope is snug and stock bolts are snug as well? Don't overdo the tightening (just snug), but loose bolts can cause accuracy issues.
 
Its been a few years so I may be off base but is the oring on the pellet probe? Firing with the bolt open wouldn't harm that. Still, replacing that oring cant hurt. As mentioned, the 'tissue test" (small piece) will tell you id there is any blowback. Rule that out and we can help with other stuff. . Don't rule out eye position. In my little world short ranges can be tougher then medium ranges depending on a host of factors.
 
I've found that consistency it more of a me than the rifle. Shooting accurately and consistently means you develop a routine, that works for you. Example

1. Rifle on bench
2. Sit behind rifle
3. Natural aim point
4. Load Pellet
5. Natural Alignment
6. Safety
7. Breathing
8. Take up slack
9. Fire
10. Follow through

Short long distances same process.

Smitty 
 
The thing I've found about benchrest shooting is it needs to be a calculated, comfortable and consistent. Although this applies mainly to benchrest shooting, I'm certain it's also a valuable part of virtually all aspects of the sport. Shot alignment/setups need to be both individual and natural. Targets need to be reacquired after every shot. Unlike speed pistol shooting , you can't push a rifle barrel into position without expecting the effects of that force and the tension required to hold the barrel on target to be evident on the outcome of the shot. I see this senario all the time. A shooter using a magazine fed gun that barely moves between shots as he/she works their way through a target card. 9 times out of 10 you can read the progression of the way these shooters have forced the gun into position after each subsequent shot by looking at the card. Even an average shooter without a polished/consistent routine will benefit greatly by moving off the gun after every shot and beginning the entire target acquisition sequence over again from the beginning.
Shoot every target as if it was the first one on the card and you'll notice an immediate improvement in the consistency of your shots.