I believe that you are looking for ‘sinon’.

Example:

Do your work, or else!

Fais ton travail, sinon!

EDIT: Since the ‘or else’ specifically refers to a threat, the below doesn’t apply.

Or:

You should do it that way, or else, it won’t work.

Tu dois le faire comme ça, sinon, cela ne marchera pas.

The translation would be “ou alors” or “sinon”. But they are very rarely used at the end of a sentence, but rather to describe an explanation/alternative/etc…

Both sinon and ou alors suit your needs, but it has to be at the end of the sentence, and you must somehow pronounce(!) the ellipsis-exclamation mark (you can use either a long-lasting sound or a very sharp stress on the very last syllable):

File-moi le chat, sinon… !

Envoie ce chat par ici, ou alors… !

The threat can as well be specified. Chiefly “ça va barder”. And sinon can also be completed with “sinon gare !”