How to slow down the video of OPPOr9 time-lapse photography?

There is no way to slow down, you can try it with third-party software.

Definition of time-lapse photography: Time-lapse photography is a kind of shooting technology that compresses time. It takes a group of photos or videos, and later, it compresses the process of minutes, hours or even days and years into a short time and plays it in the form of video. When you say slow motion, you don't mean time-lapse photography, you mean slow motion.