[LuizVaz]

Pessoal,

O CNPJ está salvo como extensão do certificado.
E está codificado na forma ASN.1.
O OID do CNPJ é 2.16.76.1.3.3.

Segue o exemplo de como recuperar o CNPJ do certificado digital:

  TfmrOID = class(TForm)
    edt1: TEdit;
    edt2: TEdit;
    edt3: TEdit;
    edt4: TEdit;
    lbl1: TLabel;
    lbl2: TLabel;
    lbl3: TLabel;
    lbl4: TLabel;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

...

const
  CAPICOM_CURRENT_USER_STORE = $00000002;
  CAPICOM_STORE_OPEN_MAXIMUM_ALLOWED = $00000002;
  CLASS_Store: TGUID = '{91D221C4-0CD4-461C-A728-01D509321556}';

//Converte Cada 2 bytes para Char
function ASN1Decode(Value: String): String;
begin
  Result := '';
  Delete(Value, 1, 4);
  repeat
    Result := Result + Char(StrToIntDef('$'+Value[1]+Value[2], 0));
    Delete(Value, 1, 2);
  until Value = '';
end;

procedure TfmrOID.Button1Click(Sender: TObject);
var
  Store        : OleVariant; //IStore3;
  Certs        : OleVariant; //ICertificates2;
  Certs2       : OleVariant; //ICertificates2;
  Cert         : OleVariant; //ICertificate2;
  Extensions   : OleVariant; //IExtensions
  Extension    : OleVariant;
  OID          : String;
  SubjectName  : TStrings;
  otherName    : TStrings;
  I            : Integer;
begin
  Try
    SubjectName := TStringList.Create;
    otherName := TStringList.Create;
    Store := CreateComObject(CLASS_Store) as IDispatch;
    if VarIsEmpty(Store) then
      Exit;
    Store.Open(CAPICOM_CURRENT_USER_STORE, 'My', CAPICOM_STORE_OPEN_MAXIMUM_ALLOWED);

    Certs := Store.Certificates;
    Certs2 := Certs.Select('Certificado(s) Digital(is) disponível(is)', 'Selecione o Certificado Digital para uso no aplicativo', false);

    if not(Certs2.Count = 0) then
    begin
      Cert := Certs2.Item[1];
      //Serial do Certificado
      edt1.Text := Cert.SerialNumber;
      //Data de Validade
      edt4.Text := Cert.ValidToDate;
      //Razão Social
      SubjectName.Text := StringReplace(Cert.SubjectName, ', ', #$D#$A, [rfReplaceAll]);
      edt3.Text := SubjectName.Values['CN'];
      //CNPJ
      edt2.Text := '';
      Extensions := Cert.Extensions;
      For i := 1 to Extensions.Count do
        begin
          Extension := Extensions.Item[i];
          OID := Extension.OID.Value;
          if (OID = '2.5.29.17') or (OID = '2.5.29.7') then
          begin
            otherName.Text := StringReplace(Extension.EncodedData.Format(true), ' ', '', [rfReplaceAll]);
            edt2.Text := ASN1Decode(otherName.Values['2.16.76.1.3.3']);
          end;
        end;
    end;
  finally
    FreeAndNil(otherName);
    FreeAndNil(SubjectName);
    Cert  := null;
    Certs2:= null;
    Certs := null;
    Store := null;
  end;
end;

Para fazer download do exemplo completo, acesse o link:
http://www.mediafire.com/file/imzmv1zmnoz/OID.rar

Att,
Luiz Vaz