Is it safe to inhale O2 from electrolysis of a pH 13.7 KOH solution?
No. Untreated and unpurified, the gas coming directly out of your reaction vessel would be unsafe to inhale. It is likely that water vapour containing traces of KOH would be present, and this poses a significant safety risk to the person breathing this vapour. You could test this yourself by measuring the pH of the headspace above your reaction vessel. Hold a piece of indicator paper here for a period, and it will slowly show a change.