I don't believe that OS has airport-specific check-in guidelines (like UA does), so I think it's just the usual T-3 hours. I don't have any experience on whether the LH desk could help you. Although seeing as they're right next to each other, I suppose you have a 50/50 chance of finding someone willing to help you provided they're not busy checking people in for the LH 417 flight to FRA at 15:30. Sorry for the non-answer..