diff --git a/core/src/utils/optionlist.h b/core/src/utils/optionlist.h
index e65261ef..d4e798d9 100644
--- a/core/src/utils/optionlist.h
+++ b/core/src/utils/optionlist.h
@@ -8,7 +8,7 @@ class OptionList {
 public:
     OptionList() { updateText(); }
 
-    void define(K key, std::string name, T value) {
+    void define(const K& key, const std::string& name, const T& value) {
         if (keyExists(key)) { throw std::runtime_error("Key already exists"); }
         if (nameExists(name)) { throw std::runtime_error("Name already exists"); }
         if (valueExists(value)) { throw std::runtime_error("Value already exists"); }
@@ -18,27 +18,27 @@ public:
         updateText();
     }
 
-    void define(std::string name, T value) {
+    void define(const std::string& name, const T& value) {
         define(name, name, value);
     }
 
-    void undefined(int id) {
+    void undefine(int id) {
         keys.erase(keys.begin() + id);
         names.erase(names.begin() + id);
         values.erase(values.begin() + id);
         updateText();
     }
 
-    void undefineKey(K key) {
-        undefined(keyId(key));
+    void undefineKey(const K& key) {
+        undefine(keyId(key));
     }
 
-    void undefineName(std::string name) {
-        undefined(nameId(name));
+    void undefineName(const std::string& name) {
+        undefine(nameId(name));
     }
 
-    void undefineValue(T value) {
-        undefined(valueId(value));
+    void undefineValue(const T& value) {
+        undefine(valueId(value));
     }
 
     void clear() {
@@ -48,61 +48,61 @@ public:
         updateText();
     }
 
-    int size() {
+    int size() const {
         return keys.size();
     }
 
-    bool empty() {
+    bool empty() const {
         return keys.empty();
     }
 
-    bool keyExists(K key) {
+    bool keyExists(const K& key) const {
         if (std::find(keys.begin(), keys.end(), key) != keys.end()) { return true; }
         return false;
     }
 
-    bool nameExists(std::string name) {
+    bool nameExists(const std::string& name) const {
         if (std::find(names.begin(), names.end(), name) != names.end()) { return true; }
         return false;
     }
 
-    bool valueExists(T value) {
+    bool valueExists(const T& value) const {
         if (std::find(values.begin(), values.end(), value) != values.end()) { return true; }
         return false;
     }
 
-    int keyId(K key) {
+    int keyId(const K& key) const {
         auto it = std::find(keys.begin(), keys.end(), key);
         if (it == keys.end()) { throw std::runtime_error("Key doesn't exists"); }
         return std::distance(keys.begin(), it);
     }
 
-    int nameId(std::string name) {
+    int nameId(const std::string& name) const {
         auto it = std::find(names.begin(), names.end(), name);
         if (it == names.end()) { throw std::runtime_error("Name doesn't exists"); }
         return std::distance(names.begin(), it);
     }
 
-    int valueId(T value) {
+    int valueId(const T& value) const {
         auto it = std::find(values.begin(), values.end(), value);
         if (it == values.end()) { throw std::runtime_error("Value doesn't exists"); }
         return std::distance(values.begin(), it);
     }
 
-    K key(int id) {
+    inline const K& key(int id) const {
         return keys[id];
     }
 
-    std::string name(int id) {
+    inline const std::string& name(int id) const {
         return names[id];
     }
 
-    T value(int id) {
+    inline const T& value(int id) const {
         return values[id];
     }
 
-    T operator[](int& id) {
-        return value(id);
+    inline const T& operator[](int& id) const {
+        return values[id];
     }
 
     const char* txt = NULL;
diff --git a/source_modules/audio_source/src/main.cpp b/source_modules/audio_source/src/main.cpp
index 5a5930e4..99efa379 100644
--- a/source_modules/audio_source/src/main.cpp
+++ b/source_modules/audio_source/src/main.cpp
@@ -25,7 +25,7 @@ ConfigManager config;
 struct DeviceInfo {
     RtAudio::DeviceInfo info;
     int id;
-    bool operator==(const struct DeviceInfo& other) {
+    bool operator==(const struct DeviceInfo& other) const {
         return other.id == id;
     }
 };
diff --git a/source_modules/hermes_source/src/hermes.h b/source_modules/hermes_source/src/hermes.h
index 96755243..0d76ea7f 100644
--- a/source_modules/hermes_source/src/hermes.h
+++ b/source_modules/hermes_source/src/hermes.h
@@ -39,7 +39,7 @@ namespace hermes {
         uint8_t gatewareVerMin;
         BoardID boardId;
 
-        bool operator==(const Info& b) {
+        bool operator==(const Info& b) const {
             return !memcmp(mac, b.mac, 6);
         }
     };
diff --git a/source_modules/spectran_source/src/main.cpp b/source_modules/spectran_source/src/main.cpp
index 129629cb..a675a286 100644
--- a/source_modules/spectran_source/src/main.cpp
+++ b/source_modules/spectran_source/src/main.cpp
@@ -503,7 +503,7 @@ private:
     float refStep = 0.5;
 
     struct SRCombo {
-        bool operator==(const SRCombo& b) {
+        bool operator==(const SRCombo& b) const {
             return baseId == b.baseId && decimId == b.decimId;
         }