Based on Brett Terpstra's Log Taskpaper Archives to Day One, except for Omnifocus.

Log to Day One.scpt

(*
Jered Benoit
jeredb.com

Omnifocus -> Day One Daily Completed Task Log

Based upon [Version 1.0] [1] of [OmniFocus - Weekly Project Report Generator] [2]
Originally Authored by Chris Brogan and Rob Trew
February 5, 2012

AND

the usual brilliance of Brett Terpstra's
[LOG TASKPAPER ARCHIVES TO DAY ONE Applescript] [3]


[1]: http://veritrope.com/code/omnifocus-weekly-project-report-generator
[2]: http://cl.ly/1H1M0S3R160x3401150u
[3]: http://brettterpstra.com/log-taskpaper-archives-to-day-one/

// NOTES

This only selects tasks completed with in the last day.

As noted in [Brett's post] [3], you must have a symbolic link to the Day One.app CLI. I dorked mine up, so there you will have to modify the noted line noted to get this scrip to work properly.

Symbolic Link terminal goodness:

> ln -s "/Applications/Day One/Day One.app/Contents/MacOS/dayone" /usr/local/bin/dayone

*)

(* 
======================================
// MAIN PROGRAM 
======================================
*)

tell application "OmniFocus"
	
	--SET THE REPORT TITLE
	set ExportList to (current date) & return & return & "Completed Projects in the Last Day" & return & "---" & return & return as Unicode text
	
	--PROCESS THE PROJECTS
	tell default document
		
		set refFolders to a reference to (flattened folders where hidden is false)
		repeat with idFolder in (id of refFolders) as list
			set oFolder to folder id idFolder
			set ExportList to ExportList & my IndentAndProjects(oFolder) & return
		end repeat
		
		--ASSEMBLE THE COMPLETED TASK LIST
		set ExportList to ExportList & return & return & "Tasks Completed in the last day" & return & "---" & return & return & return
		set day_ago to (current date) - 1 * days
		set refDoneInLastWeek to a reference to (flattened tasks where (completion date ≥ day_ago))
		set {lstName, lstContext, lstProject, lstDate} to {name, name of its context, name of its containing project, completion date} of refDoneInLastWeek
		set strText to ""
		repeat with iTask from 1 to length of lstName
			set {strName, varContext, varProject, varDate} to {item iTask of lstName, item iTask of lstContext, item iTask of lstProject, item iTask of lstDate}
			if varDate is not missing value then set strText to strText & short date string of varDate & " - "
			if varProject is not missing value then set strText to strText & " [" & varProject & "] - "
			set strText to strText & strName
			if varContext is not missing value then set strText to strText & " *@" & varContext & "*"
			set strText to strText & "  " & return
		end repeat
	end tell
	
	set ExportList to ExportList & strText as Unicode text
	
	-- Modify "/usr/local/bin/dayone/dayone" to "/usr/local/bin/dayone" if you didn't screw it up like I did.
	do shell script "echo " & (quoted form of ExportList) & "|tr -d \"\\t\"|/usr/local/bin/dayone/dayone new"
	
end tell

(* 
======================================
// MAIN HANDLER SUBROUTINES 
======================================
*)

on IndentAndProjects(oFolder)
	tell application id "OFOC"
		
		set {dlm, my text item delimiters} to {my text item delimiters, return & return}
		set day_ago to (current date) - 1 * days
		set strCompleted to (name of (projects of oFolder where its status is done and completion date ≥ day_ago)) as string
		
		set my text item delimiters to dlm
		
		return strCompleted & return
	end tell
end IndentAndProjects

ln -s "/Applications/Day One/Day One.app/Contents/MacOS/dayone" /usr/local/bin/dayone

could also be

ln -s "/Applications/Day One.app/Contents/MacOS/dayone" /usr/local/bin/dayone

me too, I found an error:
sh: /usr/local/bin/dayone: Permission denied" number 126
any help? please...

thanks, amelchi

I get a OmniFocus got an error: 2012-09-06 13:00:16.913 dayone[53738:207] No Info.plist file in application bundle or no NSPrincipalClass in the Info.plist file, exiting error.

Use dayone-cli instead:

brew cask install dayone-cli

This is the official DayOne CLI tool.