So what you're saying is, in english when I see someone faceplant and say "oooooo, that looked like it hurt", there simply isn't a way to say this in lojban. I don't feel pain so {.oiro'odai ta simlu lo ka cortu} is wrong. To translate english "ooooo" in this context, how would YOU say it john? In lojban please. For me, using lojban in everyday life, being able to assert that others UI (with the implicit pe'i) like in ".oiro'o da'oi la nik ta simlu lo ka cortu (where nick has just faceplanted)" is hella useful.
So just for clarification from other users of da'oi, am I doing it right? Or should I just start using {.oipeikau} (dunno why I hadn't thought of that before).