@WobblyHand
I have made a long tapered cone, with a polished surface, to install O-rings on small diameter shafts. The tool had a major diameter .010 " larger than the shaft and had a counterbore in the end, to receive the shaft end (for alignment and to take the chamfer out of play. I used O-ring lubricant and "screwed" the O-ring up the taper. By "screwed" I mean that I rotated the cone and used a fingernail to push the O-ring along.
If the O-ring rolls, inside out, it will likely break. If the cone doesn't work, get lower Durometer O-rings.