NOTE: while i know this is Spyfall Part 1 and the question could be answered in Spyfall Part 2, I only watch the TV series and don't read the books/comics or listen to the audio stories so there may have been a book/audio story that could answer this
in Season 12 Episode 1 Spyfall Part 1, at the end we find out that the O who we've seen so far is actually The Master who shrunk the original O to take his place.
Last we saw The Master it was as Missy and Harold Saxon on a Mondasian Colony ship where Missy stabbed Saxon and as pay back shot Missy in the back with his Laser Screwdriver. while we see Saxon descending down the elevator to his TARDIS and his regeneration process starting to turn him into Missy we know the same wouldn't happen with Missy
MASTER: No. Never. Missy! I will never stand with the Doctor!
MISSY: Yes, my dear, you will.
(So the Master zaps her in the back with his triple barrelled sonic whatever.)
MASTER: Don't bother trying to regenerate. You got the full blast.
Source: chakoteya - The Doctor Falls
now this would indicate that unless Missy was resurrected like when The Master was (or before) Yana he was resurrected by the Time Lords for the Time War or when The Master was Harold Saxon and was resurrected by his followers, The Master is dead-dead.
So do we know how this version of The Master (assuming that this one is one that comes after Missy) is alive?