Is there a way in openroad to flatten the .DEFs of a hierarchical design?

no - what is your goal behind flattening?

I want to do some STA on our multi project presentation (https://efabless.com/projects/392) I'm learning about the process, looking at what caravels `make rcx-XXXXX`does. By default it wont generate any spef for the macros. I've managed to do analysis using

sta

and

read_spef -path

but if there was a way to make a flatten .def (except the library cells) I think it would be easier to run the process The other thing that would allow us is to load Timing Report in the GUI for the whole project, to visually analyze the timing paths

It is possible to write a flattener on top of opendb but it is a bit of work. Stitching together the routes is a bit tricky. Another option is hierarchical implementation with flat signoff. I put together a doc for the caravel team - I can share it with you if you DM me your email.