The dynamic identification of structures, such as buildings and bridges, is fundamental for the assessment of structural performance, safety and maintainability, as it involves the identification of modal parameters that define the dynamic behaviour of a structure. Operational Modal Analysis (OMA) has emerged as the preferred method for this purpose, exploiting environmental vibrations without requiring external excitation. Although commercial software packages for OMA exist, they often limit user control over key settings, leading some researchers to develop customised tools to improve OMA methodologies and explore innovative identification techniques. However, the availability of stand-alone software with the versatility and advanced capabilities required to meet the different needs of practitioners and researchers is still limited. This paper presents DYMOS, a new OMA software designed to be used with environmental vibration data. DYMOS incorporates state-of-the-art algorithms for vibration-based modal identification and Optimal Sensor Placement (OSP), offering extensive customisation possibilities for practical and research applications. The second task of the software, dedicated to the OSP, allows for the execution of OSP analyses adopting as input either experimental or numerical data, the latter with the possibility to be automatically imported from a finite element software. This study describes the functionality of DYMOS and demonstrates its effectiveness in civil engineering through applications to two real-world case studies: a building with complex geometry and a newly built cable-stayed bridge.