Retrofitting office buildings plays a significant role in improving indoor conditions and promoting wellbeing and comfort. A detailed evaluation of the Indoor Environmental Quality (IEQ) is required for successful retrofitting processes. This study conducted a pre-retrofitting IEQ evaluation in existing office spaces in Oman, employing a mixed-methods approach, including a walkthrough audit. The IEQ evaluation was performed using a questionnaire and specialised instruments to evaluate thermal, visual, acoustic, and indoor air quality conditions. The measured data were compared with international standards to explore whether the internal conditions are within comfortable ranges. The IEQ subjective results revealed the need to improve the working environment, as satisfaction rates were 65% or below for the thermal, acoustical, indoor air velocity, and indoor air quality conditions. Considering IEQ objective measurements, the results indicated that thermal conditions in most offices are considered warm or hot. The visual conditions were adequate, except for some dim areas. Considering the acoustical environment, the need to reduce noise levels is obvious from the measured sound levels. The indoor air quality was evaluated by measuring the concentration levels of CO2, PM10, PM2.5, and PM1. It was revealed that blocks B and C require improvement. Several retrofitting interventions are recommended based on the study’s findings.