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Occupational Hygiene is generally defined as the art and science dedicated to the Anticipation, Recognition, Evaluation, Communication and Control of environmental stressors in, or arising from, the work place that may result in injury, illness, impairment, or affect the well-being of workers and members of the community. These stressors are normally divided into the categories Chemical, Biological, Physical, Ergonomic and Psychosocial.

A chemical hazard is a type of occupational hazard caused by exposure to chemicals in the workplace. Exposure to chemicals in the workplace can cause acute or long-term detrimental health effects. There are many types of hazardous chemicals, including neurotoxins, immune agents, dermatologic agents, carcinogens, reproductive toxins, systemic toxins, asthmagens, pneumoconiotic agents, and sensitizers.  These hazards can cause physical and/or health risks. Depending on chemical, the hazards involved may be varied.
Source: Wikipedia

Biological hazards, also known as biohazards, refer to biological substances that pose a threat to the health of living organisms, primarily that of humans. This can include samples of a microorganism, virus or toxin (from a biological source) that can affect human health. It can also include substances harmful to other animals. Source : Wikipedia

A physical hazard is an agent, factor or circumstance that can cause harm with or without contact. They can be classified as type of occupational hazard or environmental hazard. Physical hazards include ergonomic hazards, radiation, heat and cold stress, vibration hazards, and noise hazards.
Source : Wikipedia

Ergonomic hazards are physical conditions that may pose risk of injury to the musculoskeletal system, such as the muscles or ligaments of the lower back, tendons or nerves of the hands/wrists, or bones surrounding the knees, resulting in a musculoskeletal disorder (MSD). Ergonomic hazards include awkward postures, static postures, large forces, repetitive motion, or short intervals between activity. The risk of MSD is often magnified when multiple factors are present or when whole-body or hand/arm vibration, poor lighting, or poorly designed tools, equipment, or workstations produce additional negative interactions with the worker/user. Ergonomic hazards occur in both occupational and non-occupational settings such as in workshops, building sites, offices, home, school, or public spaces and facilities.
Source : Wikipedia

A psychosocial hazard is any occupational hazard that affects the psychological well-being of workers, including their ability to participate in a work environment among other people. Psychosocial hazards are related to the way work is designed, organized and managed, as well as the economic and social contexts of work and are associated with psychiatric, psychological and/or physical injury or illness. Linked to psychosocial risks are issues such as occupational stress and workplace violence which are recognized internationally as major challenges to occupational health and safety.
Source : Wikipedia

Occupational safety is about potential safety hazards that can cause injury, whereas occupational health addresses potential health concerns.