Lions head coach Sir Clive Woodward gave Wilkinson time to prove his fitness before adding his name to the 44-man squad, and Irish legend O'Reilly believes the England fly-half's kicking could prove influential in the Test series.
"If Wilkinson plays he is very important," he said. "You will get six or seven chances to have a go at their posts and he will have an 80 to 90% success rate.
"England would not have won the World Cup, even though they had some great performances from players like (Martin) Johnson and (Lawrence) Dallaglio, if they had not had Wilkinson."
The Lions could only manage a 25-25 draw with Argentina on Monday before setting off for New Zealand and O'Reilly feels the All Blacks will be tough to beat.
"When you see someone like Joe Rokocoko struggling to get on the team they must have a lot of strength," he added.
"To beat New Zealand you have to be able to take them on up front and I'd like to know what influence the Super 12 has had on New Zealand's scrummaging and tight play.
"I think they have got the strength up front and and if they can move the ball it's going to be a very fluent game played at a very high speed."