Making Music Boxes

Python 2, 95 94 bytes

^{-1 byte thanks to Value Ink}

x=1
for i in input():
 if x:o=['.']*7
 if'@'<i:o[ord(i)-65]='O'
 if'*'>i:x=i>'('
 if x:print o

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Explanation

'@'<i is to check if i is a letter, replacing the . by O on the right position.
'*'>i is to check if i is a parenthesis, if it is x=i>'(' will put 0 on x to prevent the printing/clearing of o, when i==')', it will put 1 on x re-enabling the printing/clearing of o.
When i=='.' nothing will be changed, and '.......' will be printed.
The charater order is given by their ASCII code, where '('<')'<'*'<'.'<'@'<'A'

Batch, 209 bytes

@set s=%1
@set p=)
@for %%n in (a b c d e f g)do @set %%n=.
:g
@if %s:~,1% lss @ (set "p=%s:~,1%")else set %s:~,1%=O
@set s=%s:~1%
@if %p%==( goto g
@echo %a%%b%%c%%d%%e%%f%%g%
@if not "%s%"=="" %0 %s%

Works by accumulating letters and outputting the line if the last symbol seen was not a (.

Röda, 97 78 76 bytes

{search`\(\w+\)|.`|{|c|seq 65,71|{|l|["O"]if[chr(l)in c]else["."]}_;["
"]}_}

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It's an anonymous function that reads the input from the stream. Use it like this: main { f={...}; push("ABCD") | f() }. It uses the regex from ETHproductions' answer.

Ungolfed:

{
    search(`\(\w+\)|.`) | for chord do
        seq(ord("A"), ord("G")) | for note do
            if [ chr(note) in chord ] do
                push("O")
            else
                push(".")
            done
        done
        push("\n")
    done
}

Previous answer:

f s{(s/"(?=([^()]*(\\([^()]*\\))?)*$)")|{|c|seq 65,71|{|l|["O"]if[chr(l)in c]else["."]}_;["
"]}_}

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It works by splitting the given string at places where the string following contains only matched parentheses. Then, for each chord, it iterates through possible notes and print O if the note is a member of the chord and . otherwise.