Okinawa is way too far to see or hear the explosions from Hiroshima (625 mi) or Nagasaki (460 mi) under the conditions at the time (Japan in August is incredibly humid, and the humidity is going decrease the range of sound and light). Even in ideal conditions that would be probably too far to see or hear anything for weapons of the size that they were, from the ground anyway.

In terms of knowledge of Hiroshima — I'm not sure. The return trip from Hiroshima to Tinian took 4 hours. The US announcement about the atomic bomb took place a few hours after it happened. I haven't got the will to do the time zone conversions at the moment. But it was pretty soon after it happened. The people on Tinian knew about the bombing by the time the Enola Gay landed, but I don't know if that was before or after the overall announcement by the US.

In terms of Nagasaki, Bockscar made a refueling stop at Okinawa on the way back from the bombing. On Okinawa they did tell the commanding general about their raid, because they needed his permission to get priority to communicate back to Tinian. They then ate on the island prior to returning to Tinian. It's possible they may have told others of their flight — I just don't know. In any case, by the next morning it was international news.