For oft-repeated batch assignments, you could specify the project name (and the name of its containing folder), together with the context name, at the top of a script of this general shape.
Code:
property pProjFolder : "Marketing"
property pProjName : "Research"
property pContextName : "Online"
tell application "OmniFocus"
tell front document
set oProject to first flattened project where (its name = pProjName) and (name of its folder = pProjFolder)
set oContext to first flattened context where name = pContextName
tell content of front document window
set lstTasks to (value of selected trees where class of its value is inbox task or class of its value is task)
repeat with oTask in lstTasks
set context of oTask to oContext
move oTask to end of tasks of oProject
end repeat
end tell
end tell
end tell