Hi
When you say photos on one drive, do you mean an actual photos library was stored on onedrive? Or did you export individual image files (eg jpg file) to store on one drive.
If you have individual photo files on one drive, these can simply be imported into photos (File>import)
If you have a second library that you want to merge with your system library, that is more tricky. See this user tip from Leonie for your options.
https://discussions.apple.com/docs/DOC-250000662
Or you can wait for the next version of macos which reportedly can transfer images between libraries natively within the photos app. If you are going to wait though move your library off dropbox. It is not a compatible location for a photos library, and continuing running it there will potentially damage your library causing loss of your photos.