As of now, there is no way to buy Ohm directly through Coinbase. In order to get Ohm, you will need to first purchase another cryptocurrency that is available on Coinbase, such as Bitcoin or Ethereum.
Once you have done so, you can then use that cryptocurrency to purchase Ohm through a cryptocurrency exchange.
While Coinbase does not currently offer the ability to purchase Ohm directly, they may do so in the future. For now, though, you will need to use another exchange in order to get your hands on this particular cryptocurrency.