The undertaking, which will be completed over the course of this year and next, is the next step in its ongoing OceaniaNEXT programme.
Once completed, Oceania said it would proudly deploy a fully refreshed fleet comprising brand new or "better-than-new" ships in every destination.
Oceania expects Riviera to re-enter service in December 2022 and Marina in November 2023. Each ship will accommodate 1,210 guests.
Every suite and stateroom will be renovated, while public spaces will feature a refreshed colour palette, new fabrics, furnishings and light fittings.
“At Oceania Cruises, we continue to raise the bar and elevate the guest experience at every opportunity," said president and chief executive Howard Sherman.
He added the work would extend to new culinary concepts and "immersive new destination experiences".
Riviera will debut on 7 December sailing a 16-day Mediterranean, Atlantic and Caribbean cruise from Rome to Miami.
It will then resume sailing seven- to 14-day trips from Miami to the Caribbean, Mexico and Central America before returning to Europe in late March 2023. An inaugural Far East season will follow in late 2023 and early 2024.
Marina, meanwhile, will return to service on 13 November 2023 sailing a seven-day cruise from Barcelona to Lisbon. From Lisbon, Marina will sail to Miami to sail its annual South America season, starting 3 December 2023.
Details of Oceania’s 2024 itineraries will follow in the coming weeks.
Oceania’s other ships – Insignia, Regatta, Sirena and latterly Nautica – have all been refreshed, with Nautica returning to service on 1 April.
