Learn Finnish: Lukitse ovi ja ole rauhallinen.

Lukitse comes from the Verb Type 4 verb lukita (to lock). To form the singular command for this verb type, you take the minä (I) form—minä lukitsen—and simply drop the -n at the end.

Why is the word for door ovi here instead of oven?

Usually, when you do a complete action to a whole object, the object gets an -n ending, like in minä lukitsen oven (I lock the door). However, in an imperative (command) sentence, this -n is dropped, leaving the object in its basic nominative form: lukitse ovi.

Is ole (be) also a Type 4 or 5 verb?

No, ole comes from olla (to be), which is an irregular Verb Type 3 verb. However, it follows the exact same basic rule to make a command: you take the minä form (olen) and drop the -n to get ole.

Does rauhallinen need a special ending because it is in a command sentence?

No, rauhallinen (calm) stays in its basic nominative form. This is because it is a complement describing the state of the person you are talking to, rather than being the object of an action.