OSR may refer to:

Science and technology
 Operational sex ratio, of reproductively available males to females
 On-stack replacement, used by Jikes RVM, a Java virtual machine
 Optical solar reflector, a radiator material for space craft
 OEM Service Release, a Windows 95 distribution
 Open-source robotics

Transportation
 Leoš Janáček Airport Ostrava, in Czech Republic, IATA code OSR
 Oil Spill Response, United Kingdom aircraft operator
 Ontario Southland Railway, in Canada
 Texas State Highway OSR, in the US

Other uses
 Old School Renaissance, a movement within tabletop role-playing games
 Owasippe Scout Reservation, a Boy Scout camp in Twin Lake, Michigan, U.S.
 Orchestre de la Suisse Romande, a Swiss symphony orchestra
 Operational Situation Reports, or Einsatzgruppen reports, dispatches of the Nazi death squads
 Ohio State Reformatory, a historic US prison
 Odessa Soviet Republic, a short-lived Soviet republic 
 Office of Strategic Research, a CIA intelligence analysis organization once headed by Richard Lehman
 Oilseed rape, a widely cultivated grain crop
 Overseas Service Ribbon, a US military award

See also
 OSR1, a gene
 OSR2 (gene)
 BDTH2 or OSR#1, an organosulfur compound used as a chelation agent
 OSRIC, or Old School Reference and Index Compilation, a Dungeons & Dragons retro-clone