Flow charts are not all that common for OOP-based systems, as long complex branching and looping sequences are antithetical to method-style coding.
You'd be best off doing a Google search. However, I do a lot of charting using the GraphViz utility. For general app design charting needs, I have used ArgoUML, which does a variety of chart types, though I'm not sure how actively it has been maintained. UML went from a "must-have" to relative obscurity a while back.
As I recall, Visio was pretty good at flowcharts, though not open-source. The Dia program is open-source, but I find it an inferior equivalent.
I do have a true flow-charting program, but it's in Fortran, runs on a Prime 300 minicomputer and prints best on 132-column paper (which is actually no problem with a 17.5 cpi font landscape on a laser printer, but was originally dot-matrix fanfold). Source is special comments in the Fortran code of the program to be charted. Biggest problem is that I only have it in printout form. My only portable option back then would have been paper tape.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.