I think there are different fees for diff airlines, but essentially a "no-restrictions" flight is on where you can change the date at any time w/o consequence (but these are usually so much more expensive than a regular ticket it's not worth it) and then there are flights with restrictions. I believe for most airlines you can switch the return date up to a certain period before your flight for a fee usually 1-200 dollars or so (at least I know that's what it's like for Air-canada).
If you're really unsure as to when you'll be heading back, I'd opt for the one way each way. You can usually get some pretty good last minute deals. And just as a side note, if you travel on a tuesday, it's usually the cheapest flight day so that may be something to consider.
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