Setup

org-mime exposes two functions

`org-mime-htmlize'

can be called from within a mail composition buffer to export either the entire buffer or just the active region to html, and embed the results into the buffer as a text/html mime section.

org-mime-htmlize is an interactive Lisp function in `org-mime.el'.

(org-mime-htmlize ARG)

Export a portion of an email body composed using `mml-mode' to
html using `org-mode'.  If called with an active region only
export that region, otherwise export the entire body.

`org-mime-org-buffer-htmlize'

can be called from within an Org-mode buffer to export either the whole buffer or the narrowed subtree or active region to HTML, and open a new email buffer including the resulting HTML content as an embedded mime section.

org-mime-org-buffer-htmlize is an interactive Lisp function in
`org-mime.el'.

(org-mime-org-buffer-htmlize)

Export the current org-mode buffer to HTML using
`org-export-as-html' and package the results into an email
handling with appropriate MIME encoding.

The following key bindings are suggested, which bind the C-c M-o key sequence to the appropriate org-mime function in both email and Org-mode buffers.

(add-hook 'message-mode-hook
          (lambda ()
            (local-set-key "\C-c\M-o" 'org-mime-htmlize)))

(add-hook 'org-mode-hook
          (lambda ()
            (local-set-key "\C-c\M-o" 'org-mime-org-buffer-htmlize)))