Under the agreements, Eni will own 20 percent and OMV 15 percent of the Abu Dhabi National Oil Co's ADNOC Refining, with the state firm keeping the remaining 65 percent.
The firm also announced the formation of a joint venture between the three companies to sell refined oil products on international markets, ADNOC said in a statement.
The deal values ADNOC Refining, which has a capacity of 922,000 barrels per day, at $19.3 billion, it said.
A signing ceremony in UAE capital Abu Dhabi was attended by Abu Dhabi's Crown Prince Sheikh Mohammed bin Zayed, Italy's Prime Minister Giuseppe Conte and Hartwig Loger, Austria's finance minister.
“These innovative partnerships will support our ambition of becoming an international downstream leader with the flexibility to respond quickly to shifting market needs and dynamics,” ADNOC's CEO Sultan al-Jaber said.
The deal is expected to be finalised in the third quarter of this year after securing regulatory approvals, with implementation commencing the following year.
Eni chief Claudio Descalzi said the transaction boosts the firm's global refining capacity by 35 percent, while OMV executive Manfred Leitner said on the company's website that the deal boosted its refining capacity grew by 40 percent.
Austrian Chancellor Sebastian Kurz wrote on Twitter that “today's entry by OMV into ADNOC Refining will further strengthen our extremely close bilateral relations with Abu Dhabi.”
The wealthy emirate, one of seven states making the United Arab Emirates, holds more than 90 percent of the federation's 98 billion barrels of crude oil reserves.
Last year, it renewed several key concession agreements with leading international oil majors to add further crude and natural gas production. In November, it earmarked $132 billion over the next five years to boost crude output capacity to four million barrels per day (bpd) in 2020 and to five million bpd a decade later. Its current capacity stands at 3.3 million bpd.
ADNOC last year approved $45 billion in deals to develop its downstream activities by raising refining capacity to 1.5 million bpd by 2025 and boosting its petrochemicals production.