Is 0 a decimal literal or an octal literal?

Yes, 0 is an Octal literal in C++.

As per the C++ Standard:

2.14.2 Integer literals [lex.icon]

integer-literal:  
    decimal-literal integer-suffixopt  
    octal-literal integer-suffixopt  
    hexadecimal-literal integer-suffixopt  
decimal-literal:  
    nonzero-digit  
    decimal-literal digit  
octal-literal:  
    0                           <--------------------<Here>
    octal-literal octal-digit