An example on using pTeX on Overleaf. See this help page and blog post for further details on using pTeX on Overleaf.
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)