Windows
Hold Alt and type 332 on the numeric keypad.
Accents
Copy the character, or use the shortcut that matches your keyboard and app.
All O accentsHold Alt and type 332 on the numeric keypad.
Character Viewer → o macron
Type 014C then Alt + X
Insert → Special characters → O macron
Click the keycap, then paste with Ctrl+V or Cmd+V.
Num Lock on, hold Alt, type 332, release Alt.
Use the Unicode, HTML, CSS, or JS value below in documents and stylesheets.
U+014CŌŌ\014C\u{014C}Alt + 332