diff --git a/README.md b/README.md
index 6238ce8..3525bde 100644
--- a/README.md
+++ b/README.md
@@ -69,6 +69,23 @@ Len() int
 ToSlice() []T
 ```
 
+## orderedset package
+
+This package provides an OrderedSet struct with the following methods:
+
+```go
+Add(values ...T)
+AddSlice(slice []T)
+Remove(value T)
+RemoveSlice(slice []T)
+Includes(value T) bool
+Len() int
+ToSliceFromOldest() []T
+ToSliceFromNewest() []T
+```
+
+The difference to Set is that the insertion order of the values is preserved.
+
 ## maps package
 
 Provides some helper methods for maps:
diff --git a/go.mod b/go.mod
index 501b32b..e268730 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,14 @@ module github.com/jesseduffield/generics
 
 go 1.18
 
-require golang.org/x/exp v0.0.0-20220317015231-48e79f11773a
+require (
+	github.com/wk8/go-ordered-map/v2 v2.1.8
+	golang.org/x/exp v0.0.0-20220317015231-48e79f11773a
+)
+
+require (
+	github.com/bahlo/generic-list-go v0.2.0 // indirect
+	github.com/buger/jsonparser v1.1.1 // indirect
+	github.com/mailru/easyjson v0.7.7 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/go.sum b/go.sum
index b1af872..c7577db 100644
--- a/go.sum
+++ b/go.sum
@@ -1,2 +1,18 @@
+github.com/bahlo/generic-list-go v0.2.0 h1:5sz/EEAK+ls5wF+NeqDpk5+iNdMDXrh3z3nPnH1Wvgk=
+github.com/bahlo/generic-list-go v0.2.0/go.mod h1:2KvAjgMlE5NNynlg/5iLrrCCZ2+5xWbdbCW3pNTGyYg=
+github.com/buger/jsonparser v1.1.1 h1:2PnMjfWD7wBILjqQbt530v576A/cAbQvEW9gGIpYMUs=
+github.com/buger/jsonparser v1.1.1/go.mod h1:6RYKKt7H4d4+iWqouImQ9R2FZql3VbhNgx27UK13J/0=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/josharian/intern v1.0.0/go.mod h1:5DoeVV0s6jJacbCEi61lwdGj/aVlrQvzHFFd8Hwg//Y=
+github.com/mailru/easyjson v0.7.7 h1:UGYAvKxe3sBsEDzO8ZeWOSlIQfWFlxbzLZe7hwFURr0=
+github.com/mailru/easyjson v0.7.7/go.mod h1:xzfreul335JAWq5oZzymOObrkdz5UnU4kGfJJLY9Nlc=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
+github.com/wk8/go-ordered-map/v2 v2.1.8 h1:5h/BUHu93oj4gIdvHHHGsScSTMijfx5PeYkE/fJgbpc=
+github.com/wk8/go-ordered-map/v2 v2.1.8/go.mod h1:5nJHM5DyteebpVlHnWMV0rPz6Zp7+xBAnxjb1X5vnTw=
 golang.org/x/exp v0.0.0-20220317015231-48e79f11773a h1:DAzrdbxsb5tXNOhMCSwF7ZdfMbW46hE9fSVO6BsmUZM=
 golang.org/x/exp v0.0.0-20220317015231-48e79f11773a/go.mod h1:lgLbSvA5ygNOMpwM/9anMpWVlVJ7Z+cHWq/eFuinpGE=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/maps/maps.go b/maps/maps.go
index 9d41a33..26d9c32 100644
--- a/maps/maps.go
+++ b/maps/maps.go
@@ -19,7 +19,7 @@ func Values[Key comparable, Value any](m map[Key]Value) []Value {
 func TransformValues[Key comparable, Value any, NewValue any](
 	m map[Key]Value, fn func(Value) NewValue,
 ) map[Key]NewValue {
-	output := make(map[Key]NewValue)
+	output := make(map[Key]NewValue, len(m))
 	for key, value := range m {
 		output[key] = fn(value)
 	}
@@ -27,7 +27,7 @@ func TransformValues[Key comparable, Value any, NewValue any](
 }
 
 func TransformKeys[Key comparable, Value any, NewKey comparable](m map[Key]Value, fn func(Key) NewKey) map[NewKey]Value {
-	output := make(map[NewKey]Value)
+	output := make(map[NewKey]Value, len(m))
 	for key, value := range m {
 		output[fn(key)] = value
 	}
diff --git a/orderedset/orderedset.go b/orderedset/orderedset.go
new file mode 100644
index 0000000..ed5c95b
--- /dev/null
+++ b/orderedset/orderedset.go
@@ -0,0 +1,65 @@
+package orderedset
+
+import (
+	orderedmap "github.com/wk8/go-ordered-map/v2"
+)
+
+type OrderedSet[T comparable] struct {
+	om *orderedmap.OrderedMap[T, bool]
+}
+
+func New[T comparable]() *OrderedSet[T] {
+	return &OrderedSet[T]{om: orderedmap.New[T, bool]()}
+}
+
+func NewFromSlice[T comparable](slice []T) *OrderedSet[T] {
+	result := &OrderedSet[T]{om: orderedmap.New[T, bool](len(slice))}
+	result.Add(slice...)
+	return result
+}
+
+func (os *OrderedSet[T]) Add(values ...T) {
+	for _, value := range values {
+		os.om.Set(value, true)
+	}
+}
+
+func (os *OrderedSet[T]) Remove(value T) {
+	os.om.Delete(value)
+}
+
+func (os *OrderedSet[T]) RemoveSlice(slice []T) {
+	for _, value := range slice {
+		os.Remove(value)
+	}
+}
+
+func (os *OrderedSet[T]) Includes(value T) bool {
+	return os.om.Value(value)
+}
+
+func (os *OrderedSet[T]) Len() int {
+	return os.om.Len()
+}
+
+func (os *OrderedSet[T]) ToSliceFromOldest() []T {
+	// TODO: can be simplified to
+	//   return os.om.KeysFromOldest()
+	// when we update to a newer version of go-ordered-map
+	result := make([]T, 0, os.Len())
+	for pair := os.om.Oldest(); pair != nil; pair = pair.Next() {
+		result = append(result, pair.Key)
+	}
+	return result
+}
+
+func (os *OrderedSet[T]) ToSliceFromNewest() []T {
+	// TODO: can be simplified to
+	//   return os.om.KeysFromNewest()
+	// when we update to a newer version of go-ordered-map
+	result := make([]T, 0, os.Len())
+	for pair := os.om.Newest(); pair != nil; pair = pair.Prev() {
+		result = append(result, pair.Key)
+	}
+	return result
+}
diff --git a/orderedset/orderedset_test.go b/orderedset/orderedset_test.go
new file mode 100644
index 0000000..63a39ae
--- /dev/null
+++ b/orderedset/orderedset_test.go
@@ -0,0 +1,103 @@
+package orderedset
+
+import (
+	"testing"
+
+	"github.com/jesseduffield/generics/internal/testutils"
+)
+
+func TestAddIncludes(t *testing.T) {
+	os := New[int]()
+	os.Add(1)
+	if !os.Includes(1) {
+		t.Errorf("Add(1) failed: Includes(1) returned false")
+	}
+
+	if os.Includes(0) {
+		t.Errorf("Add(1) failed: Includes(0) returned true")
+	}
+}
+
+func TestAddSliceIncludes(t *testing.T) {
+	os := New[int]()
+	os.Add(1, 2)
+	if !os.Includes(1) {
+		t.Errorf("AddSlice failed: Includes(1) returned false")
+	}
+
+	if !os.Includes(2) {
+		t.Errorf("AddSlice failed: Includes(2) returned false")
+	}
+
+	if os.Includes(3) {
+		t.Errorf("AddSlice failed: Includes(3) returned true")
+	}
+}
+
+func TestRemoveIncludes(t *testing.T) {
+	os := NewFromSlice([]int{1, 2})
+	os.Remove(1)
+	if os.Includes(1) {
+		t.Errorf("Remove failed: Includes(1) returned true")
+	}
+
+	if !os.Includes(2) {
+		t.Errorf("Remove failed: Includes(2) returned false")
+	}
+}
+
+func TestRemoveSliceIncludes(t *testing.T) {
+	os := NewFromSlice([]int{1, 2, 3})
+	os.RemoveSlice([]int{1, 2})
+	if os.Includes(1) {
+		t.Errorf("Remove failed: Includes(1) returned true")
+	}
+
+	if os.Includes(2) {
+		t.Errorf("Remove failed: Includes(2) returned true")
+	}
+
+	if !os.Includes(3) {
+		t.Errorf("Remove failed: Includes(3) returned false")
+	}
+}
+
+func TestNewFromSlice(t *testing.T) {
+	os := NewFromSlice([]int{1, 2})
+	if !os.Includes(1) {
+		t.Errorf("NewFromSlice failed: Includes(1) returned false")
+	}
+
+	if !os.Includes(2) {
+		t.Errorf("NewFromSlice failed: Includes(2) returned false")
+	}
+
+	if os.Includes(3) {
+		t.Errorf("NewFromSlice failed: Includes(3) returned true")
+	}
+}
+
+func TestLen(t *testing.T) {
+	set := NewFromSlice([]int{})
+	if set.Len() != 0 {
+		t.Errorf("Len() = %v, expected %v", set.Len(), 0)
+	}
+
+	set = NewFromSlice([]int{1})
+	if set.Len() != 1 {
+		t.Errorf("Len() = %v, expected %v", set.Len(), 1)
+	}
+}
+
+func TestToSlice(t *testing.T) {
+	set := New[int]()
+	set.Add(1)
+	set.Add(3)
+	set.Add(2)
+
+	slice := set.ToSliceFromOldest()
+	testutils.ExpectSlice(t, []int{1, 3, 2}, slice)
+
+	slice = set.ToSliceFromNewest()
+	testutils.ExpectSlice(t, []int{2, 3, 1}, slice)
+}
diff --git a/set/set.go b/set/set.go
index 0f0f4be..5b8f009 100644
--- a/set/set.go
+++ b/set/set.go
@@ -11,12 +11,9 @@ func New[T comparable]() *Set[T] {
 }
 
 func NewFromSlice[T comparable](slice []T) *Set[T] {
-	hashMap := make(map[T]bool)
-	for _, value := range slice {
-		hashMap[value] = true
-	}
-
-	return &Set[T]{hashMap: hashMap}
+	result := &Set[T]{hashMap: make(map[T]bool, len(slice))}
+	result.Add(slice...)
+	return result
 }
 
 func (s *Set[T]) Add(values ...T) {
diff --git a/slices/slices.go b/slices/slices.go
index 5bfb199..f5c1463 100644
--- a/slices/slices.go
+++ b/slices/slices.go
@@ -112,7 +112,7 @@ func MapInPlace[T any](slice []T, f func(T) T) {
 
 // Produces a new slice, leaves the input slice untouched.
 func Filter[T any](slice []T, test func(T) bool) []T {
-	result := make([]T, 0)
+	result := make([]T, 0, len(slice))
 	for _, element := range slice {
 		if test(element) {
 			result = append(result, element)
@@ -134,7 +134,7 @@ func FilterWithIndex[T any](slice []T, f func(T, int) bool) []T {
 }
 
 func TryFilter[T any](slice []T, test func(T) (bool, error)) ([]T, error) {
-	result := make([]T, 0)
+	result := make([]T, 0, len(slice))
 	for _, element := range slice {
 		ok, err := test(element)
 		if err != nil {
@@ -148,7 +148,7 @@ func TryFilter[T any](slice []T, test func(T) (bool, error)) ([]T, error) {
 }
 
 func TryFilterWithIndex[T any](slice []T, test func(T, int) (bool, error)) ([]T, error) {
-	result := make([]T, 0)
+	result := make([]T, 0, len(slice))
 	for i, element := range slice {
 		ok, err := test(element, i)
 		if err != nil {