The Mercury Seven were Alan Shepard, Gus Grissom, Gordon Cooper, Wally Schirra, Deke Slayton, John Glenn, and Scott Carpenter.
Iirc, with the Apollo 1 capsule, it wasn't just the O2 environment that was dangerous; the hatch opened inward, so it was impossible to open when the inside was pressurized. Rest in peace Gus, Ed, and Roger.