IowaSwede: Native Cedar Rapidian and now live in West Des Moines, played junior hockey myself and then ACHA college hockey at Iowa State. I know Iowa hockey well...
Well, there is obviously nothing in Ottumwa, as you know, and there are no regional leagues (otherwise I'd play in it, too). Your primary options are the following: You have adult leagues in Des Moines (Urbandale, technically), which is about 1 1/2 hours from Ottumwa. If you played in the elite J18 and/or J20, you might have to play in Des Moines' A League (given your age), although perhaps an exception could be made to allow you to play in the Sunday night B League.
Another option for you, about 1 1/2 hours away from Ottumwa, also, is adult hockey in Coralville, Iowa, at the rink in Coral Ridge Mall. About 25 minutes north of that is the Cedar Rapids Ice Arena, and the men's league in Cedar Rapids is actually pretty good. I play in it once or twice a season still when I get back to Cedar Rapids when my scouting schedule sends me through there, like it does this weekend (although I won't be playing adult league there this weekend).
Davenport has adult leagues, but that's quite a bit further than either Des Moines or Coralville or Cedar Rapids.
There's a player who now lives in Pella who played minor pro and then also hockey in Poland who makes the trek to Des Moines to play, so his situation is somewhat similar to yours.
As far as Northern Missouri goes, there's nothing. If you lived in various spots in Northern Missouri, I could tell you the rinks you'd want to skate at, but they'd be one helluva drive from Ottumwa.
If you'd like, message me and we can discuss it further. Also, given that your in Ottumwa for school, I take it your at Indian Hills? If so, what brings you to Indian Hills, and what are your plans after that? (No need to say that publicly, you can message me that info.)
I have played in all Davenport, Cedar Rapids, and Coralville and rank em this way
1. Cedar rapids best league very good arena and good league
2. Davenport (quad city sports center) were I play now another good arena but not as good as cr
3. Coralville played one session there and hated it. Ice is horrible the only league they have is full of college kids (im 23) that play for university of Iowa and most of the time I played there they didnt show and if they did most likley were drunk.
You should head up to the QC and watch the AHL flames play Im a STH there and Des moines also has an AHL team the Iowa Chops which are the affilate of Anehiem Ducks.