OmniSharp provides semantic classification of every symbol in a c# document. By default, OmniSharp-vim highlights most textual symbols, leaving language keywords and punctuation to be highlighted by Vim's standard regex-based highlighting system.

To modify OmniSharp-vim's defaults, create a g:OmniSharp_highlight_groups dictionary. The highlight groups may either be existing groups, or custom groups defined by the user:

let g:OmniSharp_highlight_groups = {
\ 'Comment': 'NonText',
\ 'XmlDocCommentName': 'Identifier',
\ 'XmlDocCommentText': 'NonText'
\}

To find out the classification name of the symbol or character under the cursor, use the :OmniSharpHighlightEcho command.

Below is the full list of available Roslyn classification type names, along with their default Vim highlight groups, as assigned by OmniSharp-vim

When Vim 8.1 text properties or neovim namespaces are not available, or when using the HTTP server, limited semantic highlighting is still possible by highlighting keywords. Note that this is not perfect - a keyword can only match a single highlight group, meaning that interfaces/classes/methods/parameters with the same name will be highlighted the same as each other.

The configuration options are also more limited. 4 highlight groups are defined for highlighting most code elements: csUserIdentifier, csUserInterface, csUserMethod and csUserType.

These highlight groups are defined as follows:

highlight default link csUserIdentifier Identifier
highlight default link csUserInterface Include
highlight default link csUserMethod Function
highlight default link csUserType Type

To change a highlight group's colors, link it to another highlight group:

highlight link csUserType Identifier

Or define the group yourself:

highlight csUserInterface ctermfg=12 guifg=Blue

To disable a group, link it to Normal:

highlight link csUserMethod Normal