This types "a".

run script "tell application \"System Events\"
" & "keystroke \"a\"" & "
end tell"

Down arrow

run script "tell application \"System Events\"
" & "keystroke \"\"" & "
end tell"

Command + O

run script "tell application \"System Events\"
" & "keystroke \"o\" using command down" & "
end tell"

Go

run script "tell application \"System Events\"
" & "keystroke \"Go\"" & "
end tell"