Just in case anyone needs clarification, by "take care of" the ground wire, I think he means loosen it so that the board can slide out. It took me a while to understand that part, but FWIW everything he said is correct.
It also gives me an idea: I'm using a cut component lead for the ground like everyone else probably, but now I'm thinking it'll be worth the effort to switch to a wire with enough slack to let the board slide out just enough to access the batteries without needing to worry about the front panel. I'm surprised I haven't read about anyone doing that in any of the build threads I read, but lots of people must have done it that way
I made mine bone-standard, but after using it for a few weeks I have a list of customizations I mean to do. Besides the ground wire, the front panel power jack bugs me way more than I expected. It makes the front look so horribly cluttered. I may do away with the gain switch as well, 2.5x is perfect for me already. Then I'm going to pull the headphone jack and replace it with a switched jack, to be connected to rear RCA jacks. That way the RCA level will be adjustable with the volume pot, and they'll switch off when headphones are inserted. The power and RCA cables will need to be given enough slack to let the board slide out of course, that detail sounds dangerously easy to forget about.
The O2 works really well in a preamp role, the alps pot I used is really clean, and I got to choose one with a linear taper that I like. I was not a fan of the front-loaded curve used on the Magni 3 pot. I thought the Magni 3 sounded great for my headphones that the o2 isn't powerful enough to drive well, but it's just not as clean sounding, which only really became apparent to me using it as a preamp.