OCD normally sends spare o-rings with new tanks. Did you save them?
If not, they do sell extra o-rings on the website for like a buck.
I agree, the IBTanked caps fit extremely tight, which is a good thing. There's really no need to remove the caps once they are in place. To clean the tanks, I lay a wash cloth in the sink and wash them in mild soapy water, rinse, and allow to dry. If necessary, you can use a Q-tip to clean inside the tank without removing the caps.
Once an end cap has been removed, be sure to lubricate the outer o-rings and rim of the glass tube with
e-liquid. Use a twisting motion to put them back on the tube. It will be difficult, but with perserverence it can be done.