Ongaro is a key part of Italy's combative pack

Ongaro, 28, has signed a three-year deal after joining from Treviso, where he won 38 Italy caps.

Johnson, who played for Biarritz in the Heineken Cup final, has committed himself for two years, but will not join up with the club until November.

Meanwhile, Saracens prop Nick Lloyd has signed a one-year contract extension.

The 29-year-old missed much of this season because of injury, but is in the Scotland A squad taking part in this summer's Churchill Cup in Canada.

Ongaro has established himself as Italy's first-choice hooker, and brings plenty of experience to Vicarage Road.

Saracens director of rugby Alan Gaffney said: "We're delighted that Fabio will be joining us from next season.

"As someone who has played in the last four Six Nations and seven Heineken Cup campaigns, he is very experienced at both international and domestic level.

"But still being only aged 28, he has a great future in the game and is very much part of our long-term planning."

Johnson will play for New Zealand NPC side Taranaki in August and September and play a series of Test matches for the Pacific Islands before joining up with Saracens.

Gaffney added: "We've met Census on a number of occasions and were very impressed with him and his performances this season in both the Heineken Cup and the French league.

"We're delighted he is joining us. He is a powerful prop who came on to the international scene last year with Samoa and, at 24, is a player for the future."