openbsd requires specifying the path for the find command. So this:

find -name \*.o
find: unknown option -- n
usage: find [-dHhLXx] [-f path] path ... [expression]

fails.

This works:

find . -name \*.o
./dist/build/Utility/Applicative.o
./dist/build/Utility/PosixFiles.o
./dist/build/Utility/Split.o
...

as expected.

Here's a patch for it: 0001-Makefile-Update-clean.patch.