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- O'Connell effect – Daniel Joseph Kelly O'Connell
- Olbers' paradox – Heinrich Wilhelm Olbers
- Ohm's law – Georg Ohm
- Okun's law – Arthur Okun
- Omori's Law – Fusakichi Omori
- Onnes effect – Heike Kamerlingh Onnes
- Oort cloud (a.k.a. Öpik-Oort Cloud) – Jan Hendrik Oort (and Ernst Julius Öpik)
- Ostwald's dilution law, Ostwald process – Friedrich Wilhelm Ostwald
- Overhauser effect – Albert Overhauser
- Ovshinsky effect – Stanford R. Ovshinsky
Read more about this topic: Scientific Phenomena Named After People