The room acoustics in the main auditorium, Store Sal, at the Oslo Opera were measured on three different occasions in 2023. The purpose was to verify that the perceived room acoustics of the hall remained unchanged after modifying the mounting of the balcony ceilings. The original reflective wooden panels were glued and had begun to detach. A new mounting solution for all the balcony ceilings was initiated by Statsbygg for safety reasons. Several room acoustic parameters were measured in octave bands and evaluated according to ISO 3382-1:2009. The measurements were carried out in February, July, and September 2023. Whilst the temperature was consistently around 20 °C on all three occasions, the relative humidity varied: 31% RH in February, 52% RH in July, and 40% RH in September. This resulted in significantly different reverberation times in the 2000 Hz, 4000 Hz, and 8000 Hz octave bands, as expected. An even greater deviation appeared at the higher frequencies when applying atmospheric normalization for air attenuation using a previous normalization method based on Harris’s correction method in one-third octave bands. The measurement system was rigorously checked, and the measurements were also repeated with a different system, resulting in the same results. This led to the conclusion that the atmospheric correction method overestimated the air attenuation. By applying a novel method for normalization in octave bands recently developed by Rindel, the measured reverberation times resulted within < 1 JND for the three occasions.