You will have to remove the seat itself first. It sits down in a pocket but only an 1/8" or so. A fine 90* pic or maybe a paper clip straightened out, one end flattened out with a hammer and then bent to a very short 90* ought to work. I have also used a 22cal (if memory serves) bore cleaning brush. Just push the brush a little ways through the seat and pull it back out. The seat should be on the brush.
Just curious what indications are you seeing that makes you think it's bad. I think this is the first time I've heard anyone specifically targeting that oring. Hope this helps.
Jimmy