Hi Roman,
You're right. Immediately after I sent the email out, I did realize that I forgot to mention my OS. I'm on Ubuntu 18.04.
I checked and I do have pd-osc installed as well. I tried [mrpeach/osc/packOSC], but it wouldn't create either. I did manage to get it working though!
I decided to see if I could find the files for this external via the terminal:
$ locate packOSC
shows:
/usr/lib/pd/extra/osc/packOSC.pd_linux
So I go into Pd and try [osc/packOSC] and it works! So apparently, Pd doesn't consider it as being a part of Mr. Peach. I wonder if anyone else can confirm that this declaration works or doesn't work for them. I would like my patches to be portable and it would be good to know if this will present any problems later.