Help please with Greek?

I'm trying to learn Greek on my own online and with books, but I'm confused about something - I listen to Greek radio online too and there are two songs that I really like and understand a few words here and there - one is 'min ta paratas, ta oneira sou' and the other is 'oneiro alithino', I know 'oneiro' means 'dream', but why does one end in 'a' and the other 'o' ?