diff --git a/crates/wasi-c2/src/error.rs b/crates/wasi-c2/src/error.rs
index f4a940283d..bb63ae6db0 100644
--- a/crates/wasi-c2/src/error.rs
+++ b/crates/wasi-c2/src/error.rs
@@ -34,6 +34,9 @@ pub enum Error {
     #[error("Unexpected IoError: {0}")]
     UnexpectedIo(#[source] std::io::Error),
 
+    #[error("Unsupported operation: {0}")]
+    Unsupported(String),
+
     // Below this, all variants are from the `$errno` type:
     /// Errno::TooBig: Argument list too long
     #[error("TooBig: Argument list too long")]
diff --git a/crates/wasi-c2/src/file.rs b/crates/wasi-c2/src/file.rs
index cbe5e2ccd5..d875be4f99 100644
--- a/crates/wasi-c2/src/file.rs
+++ b/crates/wasi-c2/src/file.rs
@@ -24,6 +24,7 @@ pub enum Filetype {
     SocketStream,
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct FdFlags {
     flags: u32,
 }
@@ -42,6 +43,7 @@ impl FdFlags {
     // etc
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct OFlags {
     flags: u32,
 }
@@ -187,10 +189,19 @@ impl WasiFile for cap_std::fs::File {
             Ok(OFlags::RDWR)
         }
     }
-    fn set_oflags(&self, _flags: OFlags) -> Result<(), Error> {
-        // XXX cap-std::fs::Permissions does not export a constructor to build this out of
-        #[allow(unreachable_code)]
-        self.set_permissions(todo!())?;
+    fn set_oflags(&self, flags: OFlags) -> Result<(), Error> {
+        #![allow(unreachable_code, unused_variables)]
+        cfg_if! {
+            if #[cfg(unix)] {
+                use std::os::unix::fs::PermissionsExt;
+                use cap_std::fs::Permissions;
+                use std::fs::Permissions as StdPermissions;
+                let flags = todo!("normalize to unix flags {:?}", flags);
+                self.set_permissions(Permissions::from_std(StdPermissions::from_mode(flags)))?;
+            } else {
+                Err(Error::Unsupported("set oflags on non-unix host system".to_owned()))
+            }
+        }
         Ok(())
     }
     fn get_filestat(&self) -> Result<Filestat, Error> {
diff --git a/crates/wasi-c2/src/snapshots/preview_1.rs b/crates/wasi-c2/src/snapshots/preview_1.rs
index d726a3dccb..731b3377b7 100644
--- a/crates/wasi-c2/src/snapshots/preview_1.rs
+++ b/crates/wasi-c2/src/snapshots/preview_1.rs
@@ -75,6 +75,7 @@ impl From<Error> for types::Errno {
             Error::DirNotCapable { .. } => Errno::Notcapable,
             Error::NotCapable => Errno::Notcapable,
             Error::TableOverflow => Errno::Overflow,
+            Error::Unsupported { .. } => Errno::Notcapable, // XXX is this reasonable?
         }
     }
 }