diff --git a/src/interfaces/libpq-oauth/Makefile b/src/interfaces/libpq-oauth/Makefile
index e90482566b1..231349034d1 100644
--- a/src/interfaces/libpq-oauth/Makefile
+++ b/src/interfaces/libpq-oauth/Makefile
@@ -24,6 +24,14 @@ override shlib := lib$(NAME)$(DLSUFFIX)
 override CPPFLAGS := -I$(libpq_srcdir) -I$(top_builddir)/src/port $(CPPFLAGS) $(LIBCURL_CPPFLAGS)
 override CFLAGS += $(PTHREAD_CFLAGS)
 
+override CPPFLAGS_SHLIB := -DUSE_DYNAMIC_OAUTH
+
+# A bit of forward-looking paranoia: don't allow libpq-oauth.so to accidentally
+# depend on the encoding IDs coming from libpq. They're not guaranteed to match
+# the IDs in use by our version of pgcommon, now that we allow the major version
+# of libpq to differ from the major version of libpq-oauth.
+override CPPFLAGS_SHLIB += -DUSE_PRIVATE_ENCODING_FUNCS
+
 OBJS = \
 	$(WIN32RES)
 
@@ -34,8 +42,7 @@ OBJS_SHLIB = \
 	oauth-curl_shlib.o \
 	oauth-utils.o \
 
-oauth-utils.o: override CPPFLAGS += -DUSE_DYNAMIC_OAUTH
-oauth-curl_shlib.o: override CPPFLAGS_SHLIB += -DUSE_DYNAMIC_OAUTH
+oauth-utils.o: override CPPFLAGS += $(CPPFLAGS_SHLIB)
 
 # Add shlib-/stlib-specific objects.
 $(shlib): override OBJS += $(OBJS_SHLIB)
diff --git a/src/interfaces/libpq-oauth/meson.build b/src/interfaces/libpq-oauth/meson.build
index 685a00acf7a..ea3a900f4f1 100644
--- a/src/interfaces/libpq-oauth/meson.build
+++ b/src/interfaces/libpq-oauth/meson.build
@@ -12,7 +12,15 @@ libpq_oauth_sources = files(
 libpq_oauth_so_sources = files(
   'oauth-utils.c',
 )
-libpq_oauth_so_c_args = ['-DUSE_DYNAMIC_OAUTH']
+libpq_oauth_so_c_args = [
+  '-DUSE_DYNAMIC_OAUTH',
+
+  # A bit of forward-looking paranoia: don't allow anyone to accidentally depend
+  # on the encoding IDs coming from libpq. They're not guaranteed to match the
+  # IDs in use by our version of pgcommon, now that we allow the major version
+  # of libpq to differ from the major version of libpq-oauth.
+  '-DUSE_PRIVATE_ENCODING_FUNCS',
+]
 
 export_file = custom_target('libpq-oauth.exports',
   kwargs: gen_export_kwargs,