AKB Conference Management event manager Ali Copeman said more than 870 old boys had registered for the reunion, many of whom arrived yesterday.
Given people's access to communications technology, she was a little surprised so many had registered, she said.
''In this climate today, I think 870 is a lot of people. It's probably one of the biggest events we'll see in Dunedin this year.
''School reunions are not as popular as they used to be because people keep in touch more on social networking sites.''
A DVML spokeswoman confirmed the reunion was the largest function the city would host this year.
The schools' two surviving rectors, Michael McMillan and Clive Rennie, will be joined by high profile former pupils, including 2011 Anzac of the Year Brigadier Dr Brian McMahon, Court of Appeal Judge Sir Bruce Robertson, theologian Sir Lloyd Geering, and The Amazing Race television producer Julian Grimmond.
Mrs Copeman said other well-known old boys including sailor Russell Coutts, rower Hamish Bond, All Black captain Ritchie McCaw and singer Jonathan Lemalu were not able to attend because of prior engagements.
The registration process begins today, and an Old Rectorians Luncheon will be held at the school hostel before assembly, roll call and a cocktail function at the Dunedin Town Hall.
It will be followed by a concert featuring the school's present pupils.
Tomorrow, the day will begin with a celebrity breakfast, with school tours with decade photographs, followed by the Otago Boys' High School 1st XV rugby match against Christ's College and the reunion dinner at the Dunedin Town Hall.
The reunion will finish on Sunday with a church service at First Church and a barbecue lunch held at the school.
