I can see strange texts on the left side of every widget like #o_intro_widget-4. Is this normal?

Don't worry, this is not a bug, they are helpers to let you know the widget ID

They only appears when you are logged in as admin, so your visitors don' see it.

This is helpfull for scrolling navigation (as you link to a widget) or when you want to create custom CSS for a targeted widget.

I found that this way was a good one to display it.

Anyway, you can hide them by adding a custom CSS in the Custom CSS tab of the Theme options:

.widgetid {display: none !important; }