Fred appears as a random encounter, with a mercenary bodyguard. When spoken to, he explains that he doesn't have anything to sell at the moment, but is on his way to Vault 81 to restock. He can often be found outside Vault 81 at the makeshift campsite with two Gunner Mercenary escorts.
He has also been found dead as the caravan leader of the ill-fated Stockton caravan, and can be found at the site with two dead mercenaries and three dead brahmins, as part of the quest Human Error.
Fred O'Connell is listed as caravan leader in the Caravan details note received from Honest Dan as part of the Human Error quest in Covenant.
If the player follows Fred after he spawns in a random event it is possible he will walk past his own dead body outside of Covonent, this is most likely an oversight by Bethesda.
If Fred and his Mercenary make it to Vault 81 (if the player follows them and keeps them alive). they will sit in front of Vault 81, however he doesn't become available as a merchant dispite him saying he went there to "restock", this is also an oversight.