OK, have tracked it down thanks to help from the list.
> Christophe Seguinot writes:
>> deleting all contents of ./build, then going through cmake, make, make install, and ldconfig.
> did you sudo make uninstall before removing the build folder (if
not they may be some remaining libgnuradio 3.8.1 librairies)
I did not sudo make uninstall before rebuilding the new version.
/usr/local/lib/x86_64-linux-gnu contained the .so for my OOT, but the symlink pointed to the old .so, not the new one that was generated.
It appears in 3.8 that OOT shared library .so symlink does not update to the newest installed version, but leaves it pointing to the older version. My recollection is that this is different behavior than gnuradio 3.7.x
I went in and manually deleted all the shared libraries for my OOT in/usr/local/lib/x86_64-linux-gnu
and then re-did sudo make install. That created the correct symlink to the new version, and the memory error problem seems to be gone.
-- Tom, N5EG