It does look like Onyx from here. Highly polished, Onxy is a bit less "glass finish shiny" than Obsidian. Can you see any light through it at all?
One way to know is to find a piece that you know is Obsidian of that same size. If this stone weighs more, it's not Obsidian. If it weighs substantially less, like almost nothing, it could be Jet. But from appearance alone, it does seem to be Onyx.