"nee and tau bullhorn"

Translation: νι και ταυ ντουντούκα

[Alex.Lupu]

I have written 'μεγαφωνο' with an omikron and it was accepted, but how do I know whether to use omikron or omega? Is there a rule, or you just have to learn the words by heart?

[Theo_Matrakas]

There isn't any rule! You have to learn it by heart, except the ending of the most verbs, which is always in omega (Ω upper case, ω lower case).

For example, αγαπώ (love), τρώω (eat) e.t.c.