Well, that's part of the difference. lol
The 'O' heads, as with all small block heads beginning with the 3418915 (J, O, U, and Z) casting on, have wider lower head bolt bosses.
Supposedly to help with head gasket sealing.
'O', BTW, some very early '68 "X" heads (2531894) weren't even cast with an 'X', but to alleviate assembly line confusion, Chrysler later had them tooled to be cast with an 'X'.
