Traditional city rivals Rangers, still smarting at their late-season collapse to finish third, allowed head coach Danny Rohl to depart for Wolfsburg and then stole Derek McInnes from runners-up Hearts in his place.
They have added seven new faces to the squad inherited by the "good Rangers man", paying out money for four of them.
Among them is last season's Premiership top scorer Lawrence Shankland, who had already agreed on his switch to Ibrox before McInnes, and goalkeeper Ivor Pandur, who has just helped Hull City return to the Premier League.
Hearts, meanwhile, have brought in 11 new players, paying transfer fees for five of them, albeit the same number of first-team players were sold.
That high turnover has hardly proved successful so far given the side now under former Sint-Truiden head coach Wouter Vrancken were thumped 4-0 away to Sturm Graz in the first leg of their opening Champions League qualifier on Tuesday.
However, they will hope that the Jamestown Analytics recruitment algorithm will again have found some hidden gems that will prove their worth in the long term.
Neighbours Hibernian suffering a surprise 2-0 defeat by Malisheva in their opening Conference League qualifier opener in Kosovo, while Motherwell's 2-0 home win over Faroe Islands part-timers HB was hardly emphatic as Scotland's hangover from another lacklustre World Cup continues.
Celtic's exits have been mainly players who spent last season exiled on loan, but they have yet to replace Kasper Schmeichel, who was still a goalkeeper of international standing before the veteran Dane's season - then career - was ended by injury.
They are poised to be further weakened by the imminent of sale to Ipswich Town of Daizen Maeda for a reported price of £10m that Chris Sutton described as "a bad deal" for the Glasgow club and a player who impressed with Japan at the World Cup.
The BBC pundit and former Celtic striker reckons the 28-year-old forward will find himself relegated from the Premier League after a season.
In addition, O'Neill is expecting two midfield sales - Maeda's international teammate, Reo Hatate, and Belgium cap Arne Engels.
The 73-year-old Northern Irishman was also forced to field awkward questions about the departure of Kelechi Iheanacho.
Celtic had the option of automatically extending the 29-year-old former Leicester City striker's stay but instead offered him a new deal.
Although O'Neill stressed that "there are two sides to every story", Iheahacho's decision to instead join Bursaspor, a club just promoted to Turkey's second tier, does not look too good.
There has also been speculation that midfield stalwart and captain Callum McGregor might be tempted to end his career with a big-money move to the Middle East.
Chief executive Michael Nicholson has admitted it has been proving "difficult" to add to O'Neill's squad because they are competing with big-spending clubs in England's top two divisions and former Celtic striker Cillian Sheridan shared his concerns on the BBC Scottish Football Podcast.
"Especially at a time when you're seeing Rangers sign players nearly every week," he said.
"It hasn't been a good window so far, which you're kind of saying every year and every six months you're talking about how they haven't had a good transfer window."
Celtic have again been strongly linked with Bodo/Glimt striker Kasper Hogh and Sheridan suggested that, like Duran, he is one who has been mentioned in dispatches for many months.
"It kind of all goes back to the fact there's no-one in the position of head of recruitment," he said.
"I just get the feeling that they're going back to players and transfer targets that were already in place before, maybe from previous people's targets."
Sheridan compared Celtic unfavourably to Motherwell, who overperformed last season as they finished fourth under new manager Jens Berthel Askou, who then won a move to Toulouse for himself and transfers to richer clubs for some of his players.
"Motherwell have a plan in place where guys are signing, they're able to sign players if there's a manager there or not," he said.
"A style of play that they want the club to play in and then a manager can come in, he'll fit with that style and he can coach whatever players are there.
"That's the modern model. Celtic are putting themselves under a lot of pressure before a ball is kicked. I just don't know how much longer Martin O'Neill will kind of just seem to go along with it for now.
"I think it's going to get to the stage where something will have to happen."
While Celtic ended last season with a league and cup double, last season's perceived poor summer transfer window led to fan protests and the departure of two managers - Brendan Rodgers and Wilfried Nancy.
"It's almost a carbon copy of last season, but actually probably worse because everyone thought they're not going to make the same mistakes," Sheridan added.
Celtic fans appear to be in agreement with their former striker.
Stephen: Michael Nicholson is clearly out his depth and needs to leave Celtic as quickly as possible. Embarrassing that he is allowing the club to peddle a narrative that the fans don't believe and that doesn't resonate with the transfer business that other clubs with far less money are doing in the UK and across Europe.
Des: Ben Gannon-Doak has proven his ability at World Cup level. But Celtic refused to pay him the £15,000 per week salary Liverpool offered him. League winners such as Maeda, Hatate and [Kyogo] Furahashi were also on low salaries, earning far less than the chief executive. Meanwhile, Celtic wasted more than £20m on transfer fees alone for third-rate wingers such as Balakwisha, Tounetki and Tillio. There's something seriously wrong at Celtic and Martin O'Neill can't fix it.
Sinclair: Doesn't matter what his [O'Neill's] priorities will be, our board will fail to deliver any of what he has been promised as they are incapable of operating in today's football world. We move so slowly on getting anything done that Martin and the fans will be disappointed again come the end of August.
Hugh: I just hope Martin O'Neill gets the full backing of our dithering board and his loyalty will not be exploited to the detriment of his legacy. He is taking a big chance and is dealing with a board that like to play hard ball whilst delivering failure after failure on the recruitment front. %!s()
