- Oort Cloud
- oort cloud
- Oort cloud
- A spherical shell of cometary bodies believed to surround the sun far beyond the orbit of Pluto and from which some are dislodged when perturbed (as by a passing star) to fall toward the sun.
- OPAG
- Outer Planets Analysis Group
- OPR
- Outer Planets Research program
- OSTP
- Office of Science and Technology Policy
- overarching
- Overarching
- Dominating or embracing all else.