[adupre]

Here's what I wrote, using AppleScript and leveraging the Omniplan API and the XMLRPC API.

It should be fairly self explanatory.
But here's the idea:
in Omniplan:
- on resources, add the custom field 'jira-user' and populate accordingly
- on tasks, add the custom field 'jira-id'. If populated, set to either the JIRA issue key or C if it is to be created by the script. (no value means it won't be handled by the script).

Open the project in question in Omniplan, run the script (from the Script Editor tool, with event log tab clicked to see the output, and potential errors).

I would recommend that you practice with a blank JIRA project :)

This script is alpha grade but I have been using it for a few weeks now and it works fine for my needs.

Feedback is welcome.
Cheers,

set login to "mylogin"
set passwd to "mypass"
set jiraProject to "PROJ"
set jiraReporter to "a_usrname"

on jiraUserForTask(_task)
	using terms from application "OmniPlan"
		set assignee to resource of item 1 of assignments of _task
		if assignee is not equal to missing value then
			set entries to custom data entries of assignee
			repeat with i from 1 to count of entries
				set entry to item i of entries
				set v to value of entry
				if name of entry is equal to "jira-user" and v is not missing value then
					return v
				end if
			end repeat
		end if
		return missing value
	end using terms from
end jiraUserForTask


on syncTaskToJira(_task, entry)
	using terms from application "OmniPlan"
		set entryKey to value of entry
		tell application "http://jira.mynetwork.com:8080/rpc/xmlrpc"
			--if assignee of issue does not equal to _task 
			set assignee to my jiraUserForTask(_task)
			if assignee is not equal to missing value then
				set t to call xmlrpc {method name:"jira1.login", parameters:{my login, my passwd}}
				
				if entryKey is equal to "C" then -- create
					set issue to call xmlrpc {method name:"jira1.createIssue", parameters:{t, {project:my jiraProject, summary:name of _task, type:"3", assignee:assignee, reporter:my jiraReporter}}}
					
					tell application "OmniPlan"
						set value of entry to |key| of issue
						set entryKey to value of entry
					end tell
				else
					set issue to call xmlrpc {method name:"jira1.getIssue", parameters:{t, entryKey}}
				end if
				
				-- Update the effort (timetracking in Jira)
				tell application "OmniPlan"
					set _effort to ((effort of _task) / (60 * 60)) as integer
					if _effort < 8 then
						set _effort to "" & _effort & "h"
					else
						set _effort to "" & ((_effort / 8) as integer) & "d"
					end if
				end tell
				
				-- Reset effort + assignee 
				set issue to call xmlrpc {method name:"jira1.updateIssue", parameters:{t, entryKey, {timetracking:{_effort}}}}
				set issue to call xmlrpc {method name:"jira1.updateIssue", parameters:{t, entryKey, {assignee:{assignee}}}}
				
			end if
		end tell
	end using terms from
end syncTaskToJira

on handleTask(_task)
	using terms from application "OmniPlan"
		--tell task _task
		set entries to custom data entries of _task
		repeat with i from 1 to count of entries
			set entry to item i of entries
			set v to value of entry
			if name of entry is equal to "jira-id" and v is not missing value then
				my syncTaskToJira(_task, entry)
			end if
		end repeat
		
		set subs to child tasks of _task
		if subs is not missing value then
			repeat with i from 1 to the count of subs
				my handleTask(item i of subs)
			end repeat
		end if
	end using terms from
end handleTask

tell application "OmniPlan"
	set _window to front window
	set _document to document of _window
	set _project to project of my _document
	set _tasks to tasks of _project
	set selTasks to selected tasks of front window
	
	
	repeat with i from 1 to the count of selTasks
		my handleTask(item i of selTasks)
	end repeat
end tell