The shortcut Alt+O for the "Ok" button in English language does not work properly.
STEPS TO REPRODUCE
1. Create a new message with composer.
2. Add an attachment to the message.
3. Right click the attachment and choose properties.
4. Place the cursor in one of the text input fields of the "Attachment Properties" dialog.
5. Press the shortcut Alt+O.
The character "o" appears at the cursor location in the text input field.
The "Attachment Properties" dialog is closed and the specified settings are applied as with pressing "Ok" with the mouse.
Linux/KDE Plasma: KDE Neon User
work fine here.