O2
Capitalize.
Not o2, O-2, or O 2.
See also P2.
object
Don’t use as a synonym for thing or item. Instead, refer to a specific object or use a contextually relevant and accurate term.
obsolete
Don’t use as a verb. Instead, use a phrase such as make obsolete.
of
Don’t use after another preposition in phrases such as off of or outside of.
offline
One word, both as an adjective and adverb. Not off line or off-line.
Use only to describe not being connected to the internet or a particular network.
off-premises, on-premises
Not off-premise and on-premise. Hyphenate.
offscreen
One word. Not off screen or off-screen.
See also onscreen.
offsite
One word. Not off site or off-site.
See also off-premises, on-premises, onsite.
OK
Not ok, okay, Okay, alright, or all right. Use uppercase.
older
Don’t use as a synonym for earlier to refer to older versions of a product or software. Instead, use earlier.
once
Use only to describe one time. Don’t use as a synonym for after or when.
online
One word, both as an adjective and adverb. Not on line or on-line.
Use to describe content or services that are available on the internet or a particular network. It is OK to use online to describe being on or connected to the internet or a particular network; but in most cases, use a contextually relevant and accurate term such as signed in or connected to the internet.
See also offline.
onscreen
One word. Not on screen or on-screen.
See also offscreen.
onsite
One word. Not on site or on-site.
See also off-premises, on-premises, offsite.
onto, on to
Use onto to refer to moving something to a position on top of something else.
Use on to when on is part of the verb.
Examples
Recommended: Drag the image onto the desktop.
See also into, in to.
open
Use open, not start or launch to describe opening a program, application, folder, or a similar target. Don’t use open for menus and commands.
Use open, not opened, to describe the open state, such as an open folder.
For more information, see [Interaction verbs]https://make.wordpress.org/docs/style-guide/developer-content/ui-elements/#open).
See also close.
open source
Two words, both as an adjective and verb. Not opensource or open-source. Use lowercase and don’t hyphenate.
option
OK to use in developer documentation and for a technical audience.
In a command-line context, use option instead of flag, argument, or parameter.
For more information, see Command-line syntax.
In general, emphasize on the task to be accomplished, rather than how the user should interact with the UI element. Refer to an option by its label. If you have to use a descriptor, use option or option button.
For more information, see UI elements.
OS, operating system
Initialism for operating system. Use uppercase for the initialism.
For the spelled out version, use two words as a noun, and hyphenate as an adjective.
For more information about spelling out abbreviations, see Abbreviations.
output
Avoid as a verb. Instead, use write to, display on, print on, print to, or a contextually relevant and accurate term.
outside
Not outside of.
over
Use to refer to a location or position above something.
In context of quantities, don’t use as a synonym for more than.
Don’t use to refer to version numbers. Instead, use later.
override
One word. Not over ride or over-ride.
overwrite
Use only to describe existing data with new data. Use replace to describe replacing an existing file with a new one that has the same name.
Don’t use as a synonym for type over.