The uni has a rule thats something along the lines of "you cant miss more than the first 2 weeks of semester due to studying abroad" if i recall correctly from my own study abroad experience that ran over time. if you are looking at an exchange that uom has listed as an exchange partner university, then uom itself will be able to tell you what the deal is with that particular place, so just ask them. if you plan your degree accordingly whereby the semester you will miss the start of is loaded up with the subjects that would most accomodatively allow you to miss a class or two early on then it should be fine. I know when i studied in America I missed the first two weeks of uni sem 2 in melb and it was fine, but on paper missing the third week would've prohibited me from the study abroad program even though I think it would've slipped through the cracks anyway (none of my classes that sem had attenddance anyway so i didnt end up going to a class until like week 4 LOL). You also have to look at the dates of your last assessments overseas and their format. I know people who studied abroad for winter/northern hemisphere summer school and even though the official dates of the course would've made them miss 3 and a half weeks of semester 2, their assignments were online submission and their last day of class was 3 weeks before the due date so they made it back in time to start semester 2 on time. just talk to the uni about it and see whats up