Here is a list of all documented functions, variables, defines, enums, and typedefs with links to the documentation:

### - o -

- OAT_256_TICKS : newgrf_animation_type.h
- OAT_BUILT : newgrf_animation_type.h
- OAT_TILELOOP : newgrf_animation_type.h
- OBJECT_CLASS_BEGIN : newgrf_object.h
- OBJECT_CLASS_MAX : newgrf_object.h
- OBJECT_FLAG_2CC_COLOUR : newgrf_object.h
- OBJECT_FLAG_ALLOW_UNDER_BRIDGE : newgrf_object.h
- OBJECT_FLAG_ANIM_RANDOM_BITS : newgrf_object.h
- OBJECT_FLAG_ANIMATION : newgrf_object.h
- OBJECT_FLAG_AUTOREMOVE : newgrf_object.h
- OBJECT_FLAG_BUILT_ON_WATER : newgrf_object.h
- OBJECT_FLAG_CANNOT_REMOVE : newgrf_object.h
- OBJECT_FLAG_CLEAR_INCOME : newgrf_object.h
- OBJECT_FLAG_DRAW_WATER : newgrf_object.h
- OBJECT_FLAG_HAS_NO_FOUNDATION : newgrf_object.h
- OBJECT_FLAG_NONE : newgrf_object.h
- OBJECT_FLAG_NOT_ON_LAND : newgrf_object.h
- OBJECT_FLAG_ONLY_IN_GAME : newgrf_object.h
- OBJECT_FLAG_ONLY_IN_SCENEDIT : newgrf_object.h
- OBJECT_HQ : object_type.h
- OBJECT_LIGHTHOUSE : object_type.h
- OBJECT_OWNED_LAND : object_type.h
- OBJECT_STATUE : object_type.h
- OBJECT_TRANSMITTER : object_type.h
- ObjectAnimationTrigger : newgrf_animation_type.h
- ObjectCallbackMask : newgrf_callbacks.h
- ObjectChangeInfo() : newgrf.cpp
- ObjectClass : newgrf_object.h
- ObjectClassID : newgrf_object.h
- ObjectFlags : newgrf_object.h
- ObjectID : object_type.h
- ObjectType : object_type.h
- OC_DEREFERENCE_POINTER : oldloader.h
- OC_END : oldloader.h
- OC_TTD : oldloader.h
- OC_TTO : oldloader.h
- OCC_EQUALS : order_type.h
- OCC_IS_FALSE : order_type.h
- OCC_IS_TRUE : order_type.h
- OCC_LESS_EQUALS : order_type.h
- OCC_LESS_THAN : order_type.h
- OCC_MORE_EQUALS : order_type.h
- OCC_MORE_THAN : order_type.h
- OCC_NOT_EQUALS : order_type.h
- OCV_AGE : order_type.h
- OCV_LOAD_PERCENTAGE : order_type.h
- OCV_MAX_SPEED : order_type.h
- OCV_RELIABILITY : order_type.h
- OCV_REMAINING_LIFETIME : order_type.h
- OCV_REQUIRES_SERVICE : order_type.h
- OCV_UNCONDITIONALLY : order_type.h
- ODATF_SERVICE_ONLY : order_type.h
- ODATFB_HALT : order_type.h
- ODATFB_NEAREST_DEPOT : order_type.h
- ODF_END : getoptdata.h
- ODF_HAS_VALUE : getoptdata.h
- ODF_NO_VALUE : getoptdata.h
- ODF_OPTIONAL_VALUE : getoptdata.h
- ODTF_MANUAL : order_type.h
- ODTFB_PART_OF_ORDERS : order_type.h
- ODTFB_SERVICE : order_type.h
- OffsetGroundSprite() : viewport.cpp , viewport_func.h
- OLD_DATA_DIR : fileio_type.h
- OLD_GM_DIR : fileio_type.h
- OldChunkType : oldloader.h
- OLF_FULL_LOAD_ANY : order_type.h
- OLF_LOAD_IF_POSSIBLE : order_type.h
- OLFB_FULL_LOAD : order_type.h
- OLFB_NO_LOAD : order_type.h
- OnChange : settings_internal.h
- OnConvert : settings_internal.h
- OnNewDay() : date.cpp
- OnNewMonth() : date.cpp
- OnNewYear() : date.cpp
- OnScreenKeyboardWidgets : osk_widget.h
- ONSF_NO_STOP_AT_ANY_STATION : order_type.h
- ONSF_NO_STOP_AT_DESTINATION_STATION : order_type.h
- ONSF_NO_STOP_AT_INTERMEDIATE_STATIONS : order_type.h
- ONSF_STOP_EVERYWHERE : order_type.h
- OnTick_Companies() : company_cmd.cpp , landscape.cpp
- OnTick_LinkGraph() : landscape.cpp , linkgraphschedule.cpp
- OPENTTD_GRAPHICS_BASE_GRF_ID : newgrf_config.cpp
- operator!=() : multimap.hpp
- operator==() : multimap.hpp
- OPFShipChooseTrack() : opf_ship.cpp , opf_ship.h
- OppositeCorner() : slope_func.h
- OptionDataFlags : getoptdata.h
- OptionMenuEntries : toolbar_gui.cpp
- OrderBackupID : order_backup.h
- OrderBackupPool : order_backup.h
- OrderConditionComparator : order_type.h
- OrderConditionCompare() : order_cmd.cpp
- OrderConditionVariable : order_type.h
- OrderDepotAction : order_type.h
- OrderDepotActionFlags : order_type.h
- OrderDepotTypeFlags : order_type.h
- OrderGoesToStation() : order_cmd.cpp
- OrderLoadFlags : order_type.h
- OrderNonStopFlags : order_type.h
- OrderStopLocation : order_type.h
- OrderType : order_type.h
- OrderTypeByte : order_type.h
- OrderUnloadFlags : order_type.h
- OrderWidgets : order_widget.h
- origin_currency_specs : currency.cpp
- ORIGINAL_BASE_YEAR : date_type.h
- ORIGINAL_END_YEAR : date_type.h
- ORIGINAL_MAX_YEAR : date_type.h
- ORIGINAL_SAMPLE_COUNT : sound_type.h
- OSK_KEYBOARD_ENTRIES : textbuf_gui.h
- OSKA_DISABLED : misc_gui.cpp
- OSKA_DOUBLE_CLICK : misc_gui.cpp
- OSKA_IMMEDIATELY : misc_gui.cpp
- OSKA_SINGLE_CLICK : misc_gui.cpp
- OskActivation : misc_gui.cpp
- OSL_PLATFORM_FAR_END : order_type.h
- OSL_PLATFORM_MIDDLE : order_type.h
- OSL_PLATFORM_NEAR_END : order_type.h
- OtherAxis() : direction_func.h
- OTTD2FS() : win32.cpp , stdafx.h
- OTTD_ALIGNMENT : endian_type.hpp
- ottd_mkdir() : strgen.cpp
- ottd_rdtsc() : debug.h , os_timer.cpp
- OTTDSHGetFolderPath() : win32.cpp , win32.h
- OTTDThreadFunc : thread.h
- OUF_UNLOAD_IF_POSSIBLE : order_type.h
- OUFB_NO_UNLOAD : order_type.h
- OUFB_TRANSFER : order_type.h
- OUFB_UNLOAD : order_type.h
- OUTPUT_BLOCK_SIZE : settingsgen.cpp
- OwnedPPPonPCP : elrail_data.h
- Owner : company_type.h
- OWNER_BEGIN : company_type.h
- OWNER_DEITY : company_type.h
- OWNER_END : company_type.h
- OWNER_NONE : company_type.h
- OWNER_TOWN : company_type.h
- OWNER_WATER : company_type.h