How do you open .mat files in Octave?

If your file contains only numbers, no .mat header, then this should work:

variable = load("filename")

Not sure which version of .mat file you might have, or whether Octave keeps up with the latest Matlab formats.

Here's a link that might be a good start.

Bottom line is that you can say: load MyMatFile.mat right at the Octave prompt.

In case you wind up having to write code to read the .mat file ever: I've done this before and it was not difficult, but was time-consuming. Mat file format for 2012a