"Oppa" and "Unnie" are titles that a girl uses for an older guy or girl respectively. Similar to in Tagalog we use "Kuya" and "Ate", but girls can say "Oppa" to a guy that they are romantically interested in, whereas in Tagalog, where the term "Kuya" from a girl you're interested in will soften the bone so fast its not even funny (since its only appropriate for non-romantic/family relationships). Guys will use the terms "Hyung" and "Noona" for an elder guy or girl respectively. If you watch enough variety shows, you pick up these things. I know there are terms for addressing younger people, I just don't know them all.
So "Oppa, Saranghaeyo" means, "Oppa, I love you."