Mick, O-rings wil work if put on as you described and if the fittings are in good enough condition.
Here are just some of my problems with them,
IF you overtighten the fitting, you can tear the O-ring
The fittings have to be in good condition and you need 2 flat surfaces that mate together pretty well.
You need to have the right size in your tool box.
However they will produce a clean looking joint, good for inside your bathroom.
Probably better than an O-ring would be a flat rubber washer, they are almost the same but will have a larger surface area to make the seal and im assuming would be harder to tear.
I think thread tape is a better solution because it will work pretty much everywhere you need to seal a thread regardless of condition and size, and if it looks a bit hairy, it can be trimmed with a stanley knife. Ive hav'nt used an o-ring or flat washer that wasnt included as part of the tap, spout etc, and have often had to replace them with tape due to my overtighting them and therefore ripping them.
My preference is, 1 thread tape, 2 flat rubber washer, 3 O-Ring, 4 LOXEAL paste (hard to undo). Hemp is my preffered joint on really big stuff.
:hpydans: