How can you argue that Oliver Cromwell was a tyrant?

To me, there are two main things you can point to in order
to argue that Cromwell was a tyrant.

First, you can look at
his tendency to keep power for himself and to oppress his enemies.  For example, he
dissolved Parliament when they did not act according to his desires.  He also used his
standing army to force local governments to act the way he wanted them to
act.

Second, I think that you can look at his actions in
Ireland.  He is criticized for the brutality of the invasion of Ireland, including
massacres like the one at Drogheda.

So here you have a
person who essentially used military force to close Parliament and to coerce local
governments.  He also (you can argue) acted brutally towards the Irish.  Both of these
ideas could be used to argue that he was a tyrant.