The AO-8 will have ample clearance for 28mm tires, probably even 32 or 35mm. In fact, until recently I ran huge 27x1-3/8" knobbies on my UO-8.
I recommend REAL 28mm tires for your application, not the 700Cx28 Continental Ultra-2000s, which are actually about 25mm wide, that I use on my Bianchi, which does not have enough clearance for real 28mm tires, such as Specialized Armadillos.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069