Wikipedia references the ratio of liquid oxygen to RP-1 as 2.56. Is this based on mass or volume? Are they using some typical combustion chamber pressure?
It's always by mass but there's one additional problem that rockets pretty much never use stoichiometric oxygen to fuel ratio (O/F) that you'd expect if you calculated molar mass of propellant components an their ideal exhaust product(s). So the source should mention that somewhere if they're using stoichiometric ratios, typical O/F or even quoting some specific system's ideal O/F (and for where, usually given for sea-level and vacuum).
As a note, quoting volumetric ratio wouldn't make sense without also quoting storage pressure and temperature. And in case of mixtures and otherwise non-pure (e.g. non-refined) propellants also their molar composition. And ullage volume. And so on.
It is a mass ratio as the ratio itself is based on the molecular masses of the fuel and oxidizer. Check out http://www.braeunig.us/space/propuls.htm and there "Combustion and Exhaust velocity" for details.