When talking about the future, they are the same. I'll buy a house one day = I will buy a house some day. 'One day' can also be used with past tenses. 'Some day' cannot.

"Someday" is less definite, "one day" implies a specific day in the future. But they can be used almost interchangeably.

Please read the answers on this page: http://english.stackexchange.com/questions/106855/what-is-the-difference-between-oneday-and-someday I believe the answers given there are correct. In addition, "one day" can also be used in a way that "someday" cannot. Besides meaning "someday," "one day" can also mean that there was only ONE day, not two, not three, etc. Example: "How long were you in Paris?" -- "I was there one day." You could not use "someday" when you are trying to specify the number of days.

Ok, "One day" is an unspecified time of either the past or future. Now because "One day" is a unified word, I'd suggest one goes with "Someday" if one is talking about some time in the future. However there is no harm in using One day, but the other adverbs should be specific of past or future tense.BUt when using Someday, it is understood as just the future.