I am having difficulty in using the grease tool to grease the bottom bracket bearings.

The tool will fit into the bottom bracket, but it is very difficult to push the tool all the way in.

It seems as though the O ring is too large and won’t locate properly.

It takes a lot of force to push the tool into place.

Any advice appreciated.

Mine pops in with a bit of a shove and some grease on the o-ring

Not had any problems with mine. I assume you have popped out the top-hats?

Yes, top hats out.

Seems as though the O ring requires a lot of pressure to sit in the right place.

Eventually with alot pressure I managed to get the grease to exit on the outboard side of the bearing.