I wonder about an oxygen mask if that would have been enough too.

That wouldn't work. It's a bit complicated, but in general terms, when you get much about about 12,000 feet, you need supplemental O2. It can be in the form of a cannula up to about 18,000 feet. Above that altitude, you need O2 under pressure. As you go higher, even that isn't enough. As some point that varies by the individual to some degree (roughly 35,000 to 45,000 feet altitude), you need pressure on the outside of the body to breathe. That has to come from either a pressurized cabin or a pressure suit. Even if the Soyuz 11 cosmonauts had oxygen masks, they still would've died because of the lack of pressure on their body. Within a short time, their blood would likely have boiled.

As for Apollo 7, if memory serves, they did wear their suits but refused to put on their helments due to their head colds. They believed they could put on their helments in the event of an emergency. Fortunately, they didn't have to find out if this was true. None of the Apollo 7 astronauts ever flew in space again. It's said that Kraft saw to that.

Supplemental Oxygen for the General Aviation Pilot

EBULLISM AT 1 MILLION FEET: Surviving Rapid/Explosive Decompression