There are multiple ways to do just about anything online. The best way to watch "Once Upon a Time" episodes online, though , on through the ABC website. This can come in handy if you are travelling, for instance, and don't have access to local channels showing the program, or there is a time change involved and watching it when it is broadcast is inconvenient.
There are other private sites which provide links to episodes or ways to view ABC in real time, such as (and I'm sure there are many others) Hulu, TV Guide, XFINITY and this one, but I don't use them and don't recommend them unless you already normally use those services or wish to use them for other purposes as well. They may be more convenient for some than the ABC portal, though, that's why I provide the links.