Phone's 3.5 mm jack is not working

Fortunately on your phone the headphone jack is on a small circuit board all by itself (well, there appears to be a microphone on it too) that can be easily replaced.

The only instructions for how to replace it I found was a YouTube video here.

Oppo F1 S Headphone Problem Solve 100% Working

The video is a little light on some of the specifics of getting the phone open, but if you use a little common sense you can get through it. This video shows fixing the port by taking it out and cleaning it thoroughly, which is certainly worth trying. If cleaning doesn't work, the part is cheap and readily available; I saw it being sold on Amazon, eBay and AliExpress; here's an example (not a recommendation).

OEM Headphone Jack Flex for OPPO F1s