Move to the beginning of line while in Insert mode

Ctrl+O whilst in insert mode puts you in command mode for one key press only. Therefore Ctrl+O then Shift+I should accomplish what you're looking for.

You could enter insert mode using I (capital i).

It will put the cursor at the beginning of the line.

Similarly you can use A to add something at the end of the line.

Though, it does not really solve the problem of moving while already being in Insert mode.

I have just checked help on Insert mode, there is no key combination in insert mode to move at the beginning of the line.

Other idea : Remap a new command only in insert mode

inoremap <C-i> <Home>