OpenGL is a graphics library only, and doesn't have (and shouldn't have) any GUI features built-in.
If you're using C++, consider using Qt for GUI-based applications.
Here's a screenshot of me using Qt to build an editor alongside my game: (still a work in progress)
Qt is for heavily GUI-driven applications, and takes over a great deal of your application. If text entry is really all you want, then Qt is way overkill, and you'd want something smaller like a OpenGL-based in-game GUI library - I don't have personal experience with any of those, but a half-dozen or so of them exist.