It's a shortened way of saying,
Oh! You wouldn't believe how much heartache I found [when I was looking for someone to love]
The interjection oh is this case serves as an intensifier to let the person know that whatever was found – in this case, heartache – was found in an extreme measure.
Another similar usage of the word oh can be found in a famous children's book by Dr. Seuss:
If the book was simply entitled "The Places You'll Go", it wouldn't have the same connotation. With the word Oh in front, the readers are clued in; it's almost as if the author is saying, "The amazing places you'll go," even with the adjective omitted.
Of course, without a specific adjective, the book could be talking about an extreme in either direction (such as, "The awful places you'll go").