Office buildings are significant contributors to energy consumption and greenhouse emissions, and it is obvious that office building occupants and their behavior play an essential role in building energy performance. However, how buildings, respectively, their indoor environment, influence building occupant´s behavior, wellbeing, and productivity is not so clear and easy to predict. The main problem is that this correlation is very subjective and influenced by many factors. This paper deals with the evaluation of subjective perception of indoor environment in two large open spaces of office buildings, each for approximately 250 persons. Based on long-term monitored data, it is possible to state that this refurbished building with high-level modern HVAC system is operated according to the best practice, respecting hygiene and technical standards related to air temperature, humidity, and CO2 concentration, however the occupant's level of complains on IEQ is relatively high. Data for evaluation of real status was collected through an anonymous questionnaire survey of occupants, focused on thermal comfort, indoor air quality, acoustics, illumination, health conditions, concentration, and psychological wellbeing. Based on the questionnaire evaluation, the paper shows how indoor environmental quality of a large office building is perceived by occupants and how this can subjectively influence not only their thermal comfort, perceived air quality, acoustics, and illumination but also their work concentration, health, and psychological wellbeing.