the first sounds somewhat like "or" [sounds is produce from mouth] the latter is more like an "Oh" [produced from your throat, and thus sounds deeper]... except the ending rises sharply like you just discover something interesting...

"ỏ" like in "hỏng" and "ỗ" like in "xin lỗi"

cách phát âm của bảng chữ cái tiếng Việt http://www.vietnameselanguage.org/language/vietnamese-alphabet.html

These words don't make sense when they stand alone like that.These are rhymes that just make sense only when combine with a consonant.Why don't you post it in any words,which contain them?