Colombian international Ospina's arrival comes amid criticism from fans at the lack of spending by new coach Carlo Ancelotti's Serie A runners-up compared to champions Juventus who splashed out 100 million euro ($117 million) to bring Real Madrid superstar Cristiano Ronaldo to Turin.
There are fears slim pickings including goalkeeper Alex Meret, midfielder Fabian Ruiz and forward Simone Verdi may not be enough for Napoli to keep pace with rivals Juventus, Inter and AC Milan.
Juventus top the summer spending with €235 million ($265 million) invested in new players — more than double rivals AS Roma and Napoli — as Serie A clubs have already spent more than €1 billion, second among the top five European leagues behind the big-spending English Premier League.
With the transfer window slamming shut in Italy at 8:00pm (1800 GMT) on Friday, Serie A teams have time to overtake the €1.037 billion spent last summer.
Ospina — who had fallen out of favour in London after the signing of Bernd Leno from Bayer Leverkusen — will cover for Meret, who broke his arm in training last month.
Gennaro Gattuso's AC Milan have had some late signings with Chelsea midfielder Tiemoue Bakayoko, Laxalt and Castillejo arriving at the San Siro. Bakayoko has agreed a season-long loan which could be turned into a permanent move.
Spaniard Castillejo, 23, reportedly agreed a deal to leave Villarreal on loan with obligation to buy for €18 million, with Carlos Bacca going the other way. Left-back Laxalt completed his Milan medical, and can officially sign from Genoa.
For Gattuso, the dream of the final hours of the market remains Lazio midfielder Sergej Milinkovic-Savic, to back up new signing Gonzalo Higuain and Patrick Cutrone.
At city rivals Inter Milan, hopes of signing Real Madrid's Croatian international midfielder Luka Modric are fading, with Roma's Radja Nainggolan and Argentina striker Lautaro Martinez among the big summer arrivals.
After the departure of Nainggolan and goalkeeper Alisson to Liverpool, Champions League semi-finalists Roma have boosted their squad with winger Justin Kluivert, midfielder Javier Pastore and French World Cup winner Steven Nzonzi from Sevilla.
“We did what we wanted, we moved quickly to have the squad straight away, then we continued to monitor the market for opportunities,” explained Roma sporting director Monchi at Nzonzi's presentation at the Stadio Olimpico on Thursday August 16th.
“I'm happy, even if you can always do better. We are not a team that sells only, we are the club that spent the third most. I've never had a “no” from the club management for the players I wanted. “We have the ambition and a plan to take Roma as high as possible.”