It's not an actual phrase with a meaning. It's more of a nonsensical, theatrical vocalization that contributes to the song's dramatic and operatic feel.
Some speculate that it might be a stylized way of saying "Oh, sweetie" or "Oh, sugar." However, there's no official confirmation from the band or Freddie Mercury himself.
The beauty of "Ooh Sukie" is that it leaves room for interpretation and adds to the song's captivating and somewhat mysterious nature.