diff --git a/stmhal/Makefile b/stmhal/Makefile
index ca697279c..9d4640d1f 100644
--- a/stmhal/Makefile
+++ b/stmhal/Makefile
@@ -73,6 +73,7 @@ SRC_C = \
 	gccollect.c \
 	pyexec.c \
 	pybmodule.c \
+	osmodule.c \
 	import.c \
 	lexerfatfs.c \
 	gpio.c \
diff --git a/stmhal/main.c b/stmhal/main.c
index 2fd5bf736..22f7be23e 100644
--- a/stmhal/main.c
+++ b/stmhal/main.c
@@ -10,7 +10,6 @@
 #include "pendsv.h"
 #include "mpconfig.h"
 #include "qstr.h"
-#include "nlr.h"
 #include "misc.h"
 #include "lexer.h"
 #include "parse.h"
@@ -23,6 +22,7 @@
 #include "gccollect.h"
 #include "pyexec.h"
 #include "pybmodule.h"
+#include "osmodule.h"
 #include "usart.h"
 #include "led.h"
 #include "exti.h"
@@ -276,6 +276,10 @@ soft_reset:
     // probably shouldn't do this, so we are compatible with CPython
     rt_store_name(MP_QSTR_pyb, (mp_obj_t)&pyb_module);
 
+    // pre-import the os module
+    // TODO don't do this! (need a way of registering builtin modules...)
+    rt_store_name(MP_QSTR_os, (mp_obj_t)&os_module);
+
     // check if user switch held (initiates reset of filesystem)
     bool reset_filesystem = false;
 #if MICROPY_HW_HAS_SWITCH
@@ -475,10 +479,8 @@ soft_reset:
 
     pyexec_repl();
 
-#if 0
     printf("PYB: sync filesystems\n");
     storage_flush();
-#endif
 
     printf("PYB: soft reboot\n");
 
diff --git a/stmhal/osmodule.c b/stmhal/osmodule.c
new file mode 100644
index 000000000..7f32fe674
--- /dev/null
+++ b/stmhal/osmodule.c
@@ -0,0 +1,178 @@
+#include <string.h>
+
+#include "nlr.h"
+#include "misc.h"
+#include "mpconfig.h"
+#include "qstr.h"
+#include "obj.h"
+#include "map.h"
+#include "systick.h"
+#include "rng.h"
+#include "storage.h"
+#include "ff.h"
+#include "osmodule.h"
+
+#if _USE_LFN
+static char lfn[_MAX_LFN + 1];   /* Buffer to store the LFN */
+#endif
+
+STATIC mp_obj_t os_listdir(uint n_args, const mp_obj_t *args) {
+    const mp_obj_type_t *local_str_type = &str_type;
+    const char *path;
+    if (n_args == 1) {
+        if (mp_obj_get_type(args[0]) == &bytes_type) {
+            local_str_type = &bytes_type;
+        }
+        path = mp_obj_str_get_str(args[0]);
+    } else {
+        path = "0:";
+    }
+
+    FRESULT res;
+    FILINFO fno;
+    DIR dir;
+#if _USE_LFN
+    fno.lfname = lfn;
+    fno.lfsize = sizeof lfn;
+#endif
+
+    res = f_opendir(&dir, path);                       /* Open the directory */
+    if (res != FR_OK) {
+        // TODO should be mp_type_FileNotFoundError
+        nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_OSError, "No such file or directory: '%s'", path));
+    }
+
+    mp_obj_t dir_list = mp_obj_new_list(0, NULL);
+
+    uint path_len = strlen(path);
+    if (path[path_len - 1] == '/') {
+        path_len--;
+    }
+
+    for (;;) {
+        res = f_readdir(&dir, &fno);                   /* Read a directory item */
+        if (res != FR_OK || fno.fname[0] == 0) break;  /* Break on error or end of dir */
+        if (fno.fname[0] == '.' && fno.fname[1] == 0) continue;             /* Ignore . entry */
+        if (fno.fname[0] == '.' && fno.fname[1] == '.' && fno.fname[2] == 0) continue;             /* Ignore .. entry */
+
+#if _USE_LFN
+        char *fn = *fno.lfname ? fno.lfname : fno.fname;
+#else
+        char *fn = fno.fname;
+#endif
+
+        if (fno.fattrib & AM_DIR) {                    /* It is a directory */
+        } else {                                       /* It is a file. */
+        }
+
+        // make a string object for this entry
+        byte *data;
+        uint fn_len = strlen(fn);
+        mp_obj_t entry_o = mp_obj_str_builder_start(local_str_type, path_len + 1 + fn_len, &data);
+        memcpy(data, path, path_len);
+        data[path_len] = '/';
+        memcpy(data + path_len + 1, fn, fn_len);
+
+        // add the entry to the list
+        mp_obj_list_append(dir_list, mp_obj_str_builder_end(entry_o));
+    }
+
+    f_closedir(&dir);
+
+    return dir_list;
+}
+
+MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(os_listdir_obj, 0, 1, os_listdir);
+
+STATIC mp_obj_t os_mkdir(mp_obj_t path_o) {
+    const char *path = mp_obj_str_get_str(path_o);
+    FRESULT res = f_mkdir(path);
+    switch (res) {
+        case FR_OK:
+            return mp_const_none;
+        case FR_EXIST:
+            // TODO should be FileExistsError
+            nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_OSError, "File exists: '%s'", path));
+        default:
+            nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_OSError, "Error creating directory '%s'", path));
+    }
+}
+
+MP_DEFINE_CONST_FUN_OBJ_1(os_mkdir_obj, os_mkdir);
+
+STATIC mp_obj_t os_remove(mp_obj_t path_o) {
+    const char *path = mp_obj_str_get_str(path_o);
+    // TODO check that path is actually a file before trying to unlink it
+    FRESULT res = f_unlink(path);
+    switch (res) {
+        case FR_OK:
+            return mp_const_none;
+        default:
+            nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_OSError, "Error removing file '%s'", path));
+    }
+}
+
+MP_DEFINE_CONST_FUN_OBJ_1(os_remove_obj, os_remove);
+
+STATIC mp_obj_t os_rmdir(mp_obj_t path_o) {
+    const char *path = mp_obj_str_get_str(path_o);
+    // TODO check that path is actually a directory before trying to unlink it
+    FRESULT res = f_unlink(path);
+    switch (res) {
+        case FR_OK:
+            return mp_const_none;
+        default:
+            nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_OSError, "Error removing directory '%s'", path));
+    }
+}
+
+MP_DEFINE_CONST_FUN_OBJ_1(os_rmdir_obj, os_rmdir);
+
+STATIC mp_obj_t os_sync(void) {
+    storage_flush();
+    return mp_const_none;
+}
+
+MP_DEFINE_CONST_FUN_OBJ_0(os_sync_obj, os_sync);
+
+STATIC mp_obj_t os_urandom(mp_obj_t num) {
+    machine_int_t n = mp_obj_get_int(num);
+    byte *data;
+    mp_obj_t o = mp_obj_str_builder_start(&bytes_type, n, &data);
+    for (int i = 0; i < n; i++) {
+        data[i] = rng_get();
+    }
+    return mp_obj_str_builder_end(o);
+}
+
+MP_DEFINE_CONST_FUN_OBJ_1(os_urandom_obj, os_urandom);
+
+STATIC const mp_map_elem_t os_module_globals_table[] = {
+    { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR_os) },
+
+    { MP_OBJ_NEW_QSTR(MP_QSTR_listdir), (mp_obj_t)&os_listdir_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_mkdir), (mp_obj_t)&os_mkdir_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_remove), (mp_obj_t)&os_remove_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_rmdir), (mp_obj_t)&os_rmdir_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_unlink), (mp_obj_t)&os_remove_obj }, // unlink aliases to remove
+
+    { MP_OBJ_NEW_QSTR(MP_QSTR_sync), (mp_obj_t)&os_sync_obj },
+
+    { MP_OBJ_NEW_QSTR(MP_QSTR_sep), MP_OBJ_NEW_QSTR(MP_QSTR__slash_) },
+
+    { MP_OBJ_NEW_QSTR(MP_QSTR_urandom), (mp_obj_t)&os_urandom_obj },
+};
+
+STATIC const mp_map_t os_module_globals = {
+    .all_keys_are_qstrs = 1,
+    .table_is_fixed_array = 1,
+    .used = sizeof(os_module_globals_table) / sizeof(mp_map_elem_t),
+    .alloc = sizeof(os_module_globals_table) / sizeof(mp_map_elem_t),
+    .table = (mp_map_elem_t*)os_module_globals_table,
+};
+
+const mp_obj_module_t os_module = {
+    .base = { &mp_type_module },
+    .name = MP_QSTR_os,
+    .globals = (mp_map_t*)&os_module_globals,
+};
diff --git a/stmhal/osmodule.h b/stmhal/osmodule.h
new file mode 100644
index 000000000..e3c13f756
--- /dev/null
+++ b/stmhal/osmodule.h
@@ -0,0 +1 @@
+extern const mp_obj_module_t os_module;
diff --git a/stmhal/qstrdefsport.h b/stmhal/qstrdefsport.h
index 5c3371dea..be30848e3 100644
--- a/stmhal/qstrdefsport.h
+++ b/stmhal/qstrdefsport.h
@@ -55,3 +55,14 @@ Q(PULL_UP)
 Q(PULL_DOWN)
 Q(PUSH_PULL)
 Q(OPEN_DRAIN)
+
+// for os module
+Q(os)
+Q(/)
+Q(listdir)
+Q(mkdir)
+Q(remove)
+Q(rmdir)
+Q(unlink)
+Q(sep)
+Q(urandom)
diff --git a/stmhal/rng.c b/stmhal/rng.c
index 20d840a6a..f0d14fe97 100644
--- a/stmhal/rng.c
+++ b/stmhal/rng.c
@@ -14,6 +14,10 @@ void rng_init(void) {
     HAL_RNG_Init(&RngHandle);
 }
 
+uint32_t rng_get(void) {
+    return HAL_RNG_GetRandomNumber(&RngHandle);
+}
+
 STATIC mp_obj_t pyb_rng_get(void) {
     return mp_obj_new_int(HAL_RNG_GetRandomNumber(&RngHandle) >> 2);
 }
diff --git a/stmhal/rng.h b/stmhal/rng.h
index 99bc5dc5f..5442d453c 100644
--- a/stmhal/rng.h
+++ b/stmhal/rng.h
@@ -1,3 +1,4 @@
 void rng_init(void);
+uint32_t rng_get(void);
 
 MP_DECLARE_CONST_FUN_OBJ(pyb_rng_get_obj);