OCT refers to a "Base Eight" number system or Octal. And DEC refers to a "Base Ten" number system or Decimal. We commonly use decimal for all our numbers. Decimal goes 0-9 and then adds the next digit (1) and starts again and so on. So 0-9 10-19-20-100-1000 etc.
Octal on the another hand goes 0-7, so eight digits. After seven it would go 10 all the way up to 17. So 0-7 10-17-20-70-100-700-1000 etc.
The number 25 in the Decimal system is equal to the number 31 in the Octal system.
There are four number systems that I know of (there is surely more.
Decimal, Base 10. 0-9
Octal, Base 8. 0-7
Binary, Base 2, 0-1
And Hexadecimal. That's base 16. 0-F. 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F After F is 10 all the way to 1F. If you thought Octal was confusing try translating Binary numbers in to Hex and back again... I had to do that for a networking course I did.