Whatbird.com

   I realize I'm basically giving nothing to work with but I didn't really know any other forum to ask this, so here's the deal;
I'm based in Southern Ontario and a week or so ago I was out in the countryside at night and heard a very strange sound that nobody could really identify; it's a very low "ohhhh ohhhhh ohhhh ohhhh" sound, sort of what I imagine a severely sick cow might sound like. It happened periodically through the night and was gone by the next day, the call wasn't ongoing and therefore hard to get a recording of. I've had the suggestion of an owl but it wasn't exaclty a "hooting" sound, more of a low moan, is there any type of nocturnal bird that could make this call? 
   Any help is appreciated!

Perhaps a loon?  They make several calls other than the "yodel" people immediately think of...

For the most part the only owl that actually makes the hooting sound is the great horned owl.  So it possible it's an owl  - there are a bunch of web sites that offer bird (owl) calls if you want to do a little surfing.  Let us know if you find it.