Marseille announced the signing of both Ocampos - who spent the second half of last season on loan at the Stade Velodrome - and former Manchester City youth player Karim Rekik.
Ocampos first arrived at Marseille in February and played 14 of the south coast club's last 15 matches in Ligue 1, starting three times as Marcelo Bielsa's men finished fourth in the French top flight.
In the first half of the 2014-15 campaign, the former River Plate winger played 17 matches for Monaco, including seven starts.
The 20-year-old leaves the principality with 10 league goals from 80 appearances, while he already has two goals and two assists for Marseille.
Rekik, a central defender, has left City after just one league appearance for the senior team, although the 20-year-old has played 54 league games on loan at PSV in his native Netherlands over the past two years.
The arrivals are welcome reinforcements for Marseille, who have already seen Giannelli Imbula, Jeremy Morel, Dimitri Payet, Andre-Pierre Gignac and Andre Ayew leave the club since the end of last season.