Why does Greek Wikipedia use the two different spellings (and pronunciations) Όθων ντε Σικόν and Οτόν ντε Σικόν for the Frankish noble Othon de Cicon?

What Billy Kerr said. To elaborate: the <Otón> transcription is a phonetic transcription from French. The <Óthōn> transcription is the longstanding traditional hellenisation of Otto; it was used inter alia for King Otto of Greece. It incorporates the –th– of the old spelling Otho; and it ends in –ōn, which makes it declinable. (In Demotic, it switches to 1st declension, and becomes <Óthōnas>.)

So the distinction between <Othōn> and <Oton> is like the distinction between, say, Christopher Columbus and Cristoforo Colombo.

I’ll add that the original Ottos that Byzantines encountered were spelled in a number of ways, including Othōn, Ōthōn, but also Ōttos, Óton, and Ónto.