Factor + math.unicode, 13 bytes

[ Σ dup .o ]

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Explanation:

It's a function that takes a sequence as input, prints its sum in octal, and returns the decimal sum as output.

• [ ... ] A quotation. An anonymous function that lives on the data stack until called or used by a combinator.
• Σ take the sum of a sequence (a string is a sequence of unicode code points)
• dup duplicate object on top of the data stack
• .o print as octal