Follow up question: Did you submit your OFII documents and have you been given an appointment yet?
As long as you have submitted your OFII documents within the first 3 months of arrival in France, you can leave and return. Technically, this has to be within the 3 months of the visa validity date, so you'd have to be back before 28 December. But this isn't always enforced, especially for US citizens.
Source: Did this myself. Had a visa which began in November, submitted my things to OFII, and then left for a conference without an OFII appointment and without a residence permit. In my case, I got a letter in the mail for an appointment, which I missed due to being out of the country. It was still fine.
Edited to add: Bring any paperwork you have with you relating to your upcoming OFII appointment and visa situation, and present it at the border if you are asked. Your visa stops being valid 3 months after it begins if you do not have it validated via the OFII process. So be sure that you arrive before the 28th December.
The worst case scenario would be that you have to return home and request a re-entry visa. On the books, this is something which can happen. But I don't think it's very likely because 1) you have an upcoming OFII appointment; 2) you are a US citizen and have visa requirements waived; 3) less-valuable anecdodal experience of myself, who didn't even have an OFII appointment I knew about, missed that appointment, and still had no problems.
While you are away, be sure to check the requirements of all the documents you need for the OFII appointment and prepare them beforehand.