Don’t make noise here.
Interesting: “väsen” already means “noise”, apparently. And “oväsen” is even more disturbing noise than “väsen”.
Usually, a word prefixed with “o” means the word’s opposite, I believe.
Does somebody know the etymology of “oväsen”?
The prefix “o” can sometimes reinforce the meaning, mostly for the worse, like here. There is a similar case with “ljud” and “oljud”.
In northern dialects, “o” can also be used with participles: “Jag har obäddat sängen” (I haven’t made the bed yet). Even in standard Swedish: “Jag har några saker ogjorda” (I have some things left to do).
By the way, this sentence could be reformulated using deponens: “Väsnas inte här.”.