To me that bit is to tight a fit....it barely fits now.
When you pit the guards in place the little extra space taken up now pinches the mouth to to tight a fit.
See for this reason I do not like o-rings for my horses.
I look at it as soon as you add bit guards you did away with the reasoning behind the o-ring in the first place...so why not use a eggbutt, d-ring and have a different bit end that does not create a pinch surface chance...
I understand the reasons behind o-rings but to me it defeats the purpose when bit guards are now in place..
To use a bit measure inside edge of ring to inside edge of ring is putting the mouth edge in harms way.
To use a bit that is measured to inside edge of a ring that is not hole drilled through and flexing piece of steel...it is a poor description but for me to use that o-ring with bit guards would have that horse in a minimum of 1/4" -1/2" larger mouthpiece to protect that soft tissue from rubs and pinches even with bit guards in place.
I'll try to describe it better/clearer as I think it...
So put the bit guards in place...stretch the bit and measure the inside of bit guard to inside edge of bit guard and what do you have.
If a 5" bit you only have 4.75 of space for the mouth.
If you had a 5.25 or 5.5" you would have 5" of space for the mouth.