o-umlaut is indeed oe (like in (ich) hore and bose, except with the umlauts of course). None of those really sound like the u in burst (well maybe hure would sound close to hore, but I can't say the same for buse/bose).

Just so we're on the same page, the 'u' in burst is often pronounced almost like an 'e' (yes, berst ), pronouncing it with a 'u' is not as common unless you're emphasizing the word (e.g., BURSTING through the gate).

I can't really think of a word in English that has this sound in it.