I would take more notice of the accents over the 'e' if I could see them better! Unfortunately my eyesight is not what it was and I can't see any difference! I can on the above answer as they are much bigger.
Are you referring to "o" and "oppure" ? If you are, there is one more "ovvero" :)
There is no grammatical difference between "o" and "oppure".
However if you use oppure than you are stressing the idea of choice, while by using "o" you're a bit more indifferent whether you'll choose tea or coffee.
On other hand "ovvero" would translate to "or rather"