Ohhhh sneaky. . .
Oct = octal, which is a base 8 numbering system and Dec = decimal, which is base 10.
It looks like this:
Octal numeration: 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13 ,14, 15, 16, 17, 20, 21, 22, 23, 24,25, 26, 27, 30, 31
Decimal numeration: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25