diff --git a/src/output/plugins/OssOutputPlugin.cxx b/src/output/plugins/OssOutputPlugin.cxx
index 3e65f56b7..b895046f0 100644
--- a/src/output/plugins/OssOutputPlugin.cxx
+++ b/src/output/plugins/OssOutputPlugin.cxx
@@ -421,6 +421,7 @@ oss_setup_sample_rate(int fd, AudioFormat &audio_format,
  * Convert a MPD sample format to its OSS counterpart.  Returns
  * AFMT_QUERY if there is no direct counterpart.
  */
+gcc_const
 static int
 sample_format_to_oss(SampleFormat format)
 {
@@ -459,6 +460,7 @@ sample_format_to_oss(SampleFormat format)
  * Convert an OSS sample format to its MPD counterpart.  Returns
  * SampleFormat::UNDEFINED if there is no direct counterpart.
  */
+gcc_const
 static SampleFormat
 sample_format_from_oss(int format)
 {