Can cleveref be made to use the Oxford comma for multiple citations?

Question

Simple question: Is there a simple way to get cleveref to use the Oxford comma?

\documentclass{article}
\usepackage[standard]{ntheorem}
\usepackage{cleveref}

\begin{document}

\begin{proposition}\label{thm:roses}
  Roses are red.
\end{proposition}

\begin{proposition}\label{thm:violets}
  Violets are blue.
\end{proposition}

\begin{proposition}
  42.
\end{proposition}

\begin{proposition} \label{thm:orchids}
  Orchids are orchid.
\end{proposition}

\begin{theorem}
  There exist flowers in at least three different colors.
\end{theorem}
\begin{proof}
  Immediate from \cref{thm:roses,thm:violets,thm:orchids}.
\end{proof}

\end{document}

Should say “propositions 1, 2, and 4.”

Answer 1

If you define this new command

\newcommand{\creflastconjunction}{, and\nobreakspace}

you will get Oxford comma everywhere you use multiple references like those.

MWE:

\documentclass{article}
\usepackage[standard]{ntheorem}
\usepackage{cleveref}

\newcommand{\creflastconjunction}{, and\nobreakspace}

\begin{document}

\begin{proposition}\label{thm:roses}
  Roses are red.
\end{proposition}

\begin{proposition}\label{thm:violets}
  Violets are blue.
\end{proposition}

\begin{proposition}
  42.
\end{proposition}

\begin{proposition} \label{thm:orchids}
  Orchids are orchid.
\end{proposition}

\begin{theorem}
  There exist flowers in at least three different colors.
\end{theorem}
\begin{proof}
  Immediate from \cref{thm:roses,thm:violets,thm:orchids}.
\end{proof}

\end{document} 

Output:

---

ADDENDUM

If you were wondering why you have to use \newcommand instead of renewcommand, this is why.

cleveref defines, at the beginning of the document, a lot of commands depending on the language. If you don't specify any language, english is loaded. This is the relevant part of cleveref.sty:

\DeclareOption{english}{%
  \AtBeginDocument{%
    ....
    \def\creflastconjunction@preamble{ and\nobreakspace}%
    ....

Also, you can find the following lines

\AtBeginDocument{%
  ....
  \@ifundefined{creflastconjunction}{%
    \let\creflastconjunction\creflastconjunction@preamble%
  }{%
  ....
  }%

which, at the beginning of the document, assign to \creflastconjunction the meaning of \creflastconjunction@preamble when it has not been defined yet.

In other words, \creflastconjunction gets defined only after \begin{document}. In fact, if you try to put the line

\newcommand{\creflastconjunction}{, and\nobreakspace}

inside the document, you will get an error. In this case, you would have written

\renewcommand{\creflastconjunction}{, and\nobreakspace}

Answer 2

Simple answer: You can use

\newcommand{\creflastconjunction}{, and~}

From the manual:

\creflastconjunction is used between the penultimate and final cross-reference in a list of more than two [pg. 12, at least for the 2012-03-07 version of the manual I have]

---

Redoing your example:

\documentclass{article}
\usepackage\[standard\]{ntheorem}
\usepackage{cleveref}

\newcommand{\creflastconjunction}{, and~}

\begin{document}

\begin{proposition}\label{thm:roses}
  Roses are red.
\end{proposition}

\begin{proposition}\label{thm:violets}
  Violets are blue.
\end{proposition}

\begin{proposition}
  42.
\end{proposition}

\begin{proposition} \label{thm:orchids}
  Orchids are orchid.
\end{proposition}

\begin{theorem}
  There exist flowers in at least three different colors.
\end{theorem}
\begin{proof}
  Immediate from \cref{thm:roses,thm:violets,thm:orchids}.
\end{proof}

\end{document}

Answer 3

You can use \crefmultiformat:

\documentclass{article}
\usepackage[standard]{ntheorem}
\usepackage{cleveref}

\crefmultiformat{proposition}{propositions~#2#1#3}%
  { and~#2#1#3}{, #2#1#3}{, and~#2#1#3}

\begin{document}

\begin{proposition}\label{thm:roses}
  Roses are red.
\end{proposition}

\begin{proposition}\label{thm:violets}
  Violets are blue.
\end{proposition}

\begin{proposition}
  42.
\end{proposition}

\begin{proposition} \label{thm:orchids}
  Orchids are orchid.
\end{proposition}

\begin{theorem}
  There exist flowers in at least three different colors.
\end{theorem}
\begin{proof}
  Immediate from \cref{thm:roses,thm:violets,thm:orchids}.
  Immediate from \cref{thm:roses,thm:violets}.
\end{proof}

\end{document}