Octal equivalent in C#

No, there are no octal number literals in C#.

For strings: Convert.ToInt32("12", 8) returns 10.

No there isn't, the language specification (ECMA-334) is quite specific.

4th edition, page 72

9.4.4.2 Integer literals

Integer literals are used to write values of types int, uint, long, and ulong. Integer literals have two possible forms: decimal and hexadecimal.

No octal form.

No, there are no octal literals in C#.

If necessary, you could pass a string and a base to Convert.ToInt32, but it's obviously nowhere near as nice as a literal:

int i = Convert.ToInt32("12", 8);