As of 2015, one place to watch old episodes of "Once Upon A Time" online is the TV Fanatic website. The site is free to use and does not require registration.
The TV Fanatic site offers several seasons of the show "Once Upon A Time" as well as other popular television series. The site also offers a free app so programs can be viewed on mobile devices.
Another good location where episodes of "Once Upon A Time" can be viewed online is Xfinity. This site is operated by Comcast Cable. Previews and highlights of the show can also be found on this site.