Absolutely. When Apollo 13 had their, um “problem,” they had plenty of oxygen on board to make the return trip, in spite of one of their tanks exploding. However, the expected buildup of CO2 in the lunar module (where they planned to hunker down for the return trip) was a major concern. The LM was supposed to keep two people alive for two days, not three people for four days. The command module had enough chemical “scrubber” canisters for removing CO2, but they were designed for the command module and didn’t fit the system in the lunar module. In the end, they had to fabricate an adapter so they could fit the CM’s square canisters to the LM’s round sockets. They didn’t transfer back to the CM until shortly before reentry.
A CO2 concentration of 8% in air appears to be enough to render a person unconscious (possibly dead), even though such a mix would certainly contain enough oxygen to sustain life.