diff --git a/cli_args_test.go b/cli_args_test.go
new file mode 100644
index 0000000..844a0b1
--- /dev/null
+++ b/cli_args_test.go
@@ -0,0 +1,52 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+type testcase struct {
+	// i is the input
+	i []string
+	// o is the expected output
+	o []string
+}
+
+func TestPrefixOArgs(t *testing.T) {
+	testcases := []testcase{
+		testcase{
+			i: nil,
+			o: nil,
+		},
+		testcase{
+			i: []string{"gocryptfs"},
+			o: []string{"gocryptfs"},
+		},
+		testcase{
+			i: []string{"gocryptfs", "-v"},
+			o: []string{"gocryptfs", "-v"},
+		},
+		testcase{
+			i: []string{"gocryptfs", "foo", "bar", "-v"},
+			o: []string{"gocryptfs", "foo", "bar", "-v"},
+		},
+		testcase{
+			i: []string{"gocryptfs", "foo", "bar", "-o", "a"},
+			o: []string{"gocryptfs", "-a", "foo", "bar"},
+		},
+		testcase{
+			i: []string{"gocryptfs", "foo", "bar", "-o", "a,b,xxxxx"},
+			o: []string{"gocryptfs", "-a", "-b", "-xxxxx", "foo", "bar"},
+		},
+		testcase{
+			i: []string{"gocryptfs", "foo", "bar", "-oooo", "a,b,xxxxx"},
+			o: []string{"gocryptfs", "foo", "bar", "-oooo", "a,b,xxxxx"},
+		},
+	}
+	for _, tc := range testcases {
+		o := prefixOArgs(tc.i)
+		if !reflect.DeepEqual(o, tc.o) {
+			t.Errorf("\n   i=%q\nwant=%q\n got=%q", tc.i, tc.o, o)
+		}
+	}
+}