I emailed this question to the Star-K and received the following reply:
“Hello,
Some time ago Rabbi Heinemann checked a OneBlade shaver and concluded that it was not razor sharp, and therefore paskened that it could be used by someone who shaves. However, he recently checked another One Blade, and felt that this one was razor sharp. Possibly, there is some variation in the OneBlades. Due to these contradictory results, Rabbi Heinemann has said that each OneBlade needs to be tested individually to determine whether it is razor sharp. As testing a OneBlade will render it unusable, there is no practical way to test each OneBlade.
Even if we could determine that a OneBlade is razor sharp, that does not necessarily mean that it is forbidden. Rabbi Heinemann has said that a razor sharp blade is forbidden only if it cuts close to the skin. If the blade leaves hair long enough that the stubble can be felt when moving one’s hand over the surface of the skin in the direction of the grain of the hair, it is muttar to use. It is not sufficient that the stubble can be felt when moving one’s hand against the grain of the hair. Tests with the OneBlade regarding whether stubble can be felt when moving one’s hand in the direction of the grain of the hair were inconclusive.
Therefore, Rabbi Heinemann paskens that it is inconclusive whether the OneBlade shaver is permitted or not. As a result of this uncertainty, the OneBlade shaver should not be used.”