Oceanography is the study of the physical and biological aspects of the ocean.

The Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate.

An oceanic climate, also known as a maritime climate, and generally features mild summers (relative to their latitude) and cool but not cold winters, with a relatively narrow annual temperature range and few extremes of temperature.

Ocean Thermal Energy Conversion uses the ocean thermal gradient between cooler deep and warmer shallow or surface seawaters to run a heat engine and produce useful work, usually in the form of electricity.

Ocean heat content (OHC) is a term for the energy absorbed by the ocean, which is stored as internal energy or enthalpy.

Ocean floor disposal is a method of sequestering radioactive waste in ocean floor sediment where it is unlikely to be disturbed either geologically or by human activity.

Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere.

The Occupational Safety and Health Administration (OSHA) is a large regulatory agency of the United States Department of Labor.

Occupational safety and health (OSH), also commonly referred to as occupational health and safety (OHS) is a multidisciplinary field concerned with the safety, health, and welfare of people at occupation.

Occupational hygiene (United States: industrial hygiene) is the anticipation, recognition, evaluation, control, and confirmation of protection from hazards at work that may result in injury, illness, or affect the well being of workers.

An occupational exposure limit is an upper limit on the acceptable concentration of a hazardous substance in workplace air for a particular material or class of materials.

Occupational exposure banding, also known as hazard banding, is a process intended to quickly and accurately assign chemicals into specific categories (bands), each corresponding to a range of exposure concentrations designed to protect worker health.

Observation is the active acquisition of information from a primary source.

Operation and maintenance (O&M)

Numerical weather prediction (NWP) uses mathematical models of the atmosphere and oceans to predict the weather based on current weather conditions.

A nuclide (or nucleide) is a class of atoms characterized by their number of protons, Z, their number of neutrons, N, and their nuclear energy state.

Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei.

A nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines an isotope's mass number (nucleon number).

Nucleation is the first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organization.

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element.