I used to fill then rest the cap on top and cap them all after I was done. After you fill, you will release a little bit of CO2. Since its heavier than O2, you will start to push the oxygen out, and not cap it in the bottle.
I do it the same way as Chimone,
Fill a bottle and set it on the counter and set the cap on it,
than do the next one.
Yesterday I filled the whole batch before I tightened any caps,
Then tightened them all.
I suspected gasses would force the oxygen out from under the cap before it was tightened and now I have evidence that it does.
Thanks Chimone Dude,
Knife