Here is a link to a tract entitled 'Arbitrary government displayed to the life, in the tyrannic usurpation of a junto of men called the Rump Parliament, and more especially in that of the tyrant and usurper, Oliver Cromwell in which you have a clear view of the arbitrary, illegal, and unjust proceedings, of those persons under the notion of liberty, and a compendious history of those times, faithfully collected : with the characters and lives of several of those usurpers, and a brief account of the several persons that suffered death, and imprisonment under them for their loyalty to their king and cuontry [sic] ... : illustrated with several brass cutts, representing the chief persons and passages therein.'

It was written by Thomas May, whom I believe was a Royalist Member of Parliament, in 1682.

A really quick search gets me a few examples. London's triumphant holiday by someone called Charles Hammond (though I'm not aware who he is and he doesn't seem to be in the ODNB). The subtitle of this calls Cromwell a 'bloodthirsty tyrant'. The anonymously authored Vengeance against bloody murtherers from 1661 calls Cromwell 'a Grand Usurper and Tyrant'. From 1683 there is Arbitrary government display'd to the life, in the tyrannic usurpation of a junto of men called the Rump Parliament, and more especially in that of the tyrant and usurper, Oliver Cromwell. by Thomas May, who was MP for Chichester.

Honestly though, there will be loads of sources that say this I'd bet. I don't really work on Cromwell but I'm very familiar with Restoration-era writing about other Protestant groups and I know that comparing people to Cromwell and his regime was a common way of implying other people sought to impose tyranny, so there will certainly be plenty of texts discussing Cromwell in a similar way. Do you know if you have any access via a school or college to JISC historical texts or Early English Books Online? Those are the best way to find things like this.