Example using xr package on Overleaf
This project demonstrates how the xr package can be used on Overleaf to resolve labels in other .tex files that have not been \include-d in the main file.
Make sure you enable the correct system call in latexmkrc, depending on the LaTeX engine you're using!
LianTze Lim (from an answer by cyberSingularity at http://tex.stackexchange.com/a/69832/226)
An example on using pTeX on Overleaf. See this help page and blog post for further details on using pTeX on Overleaf.