Note that OlyZaps for bonds is still being built out; you can only zap into sOHM for now.
OlyZaps combines a few transactions (token approval and swap) into a single transaction. As a result, there is less exposure to gas price volatility and the gas fees you end up paying should be the same or slightly less than performing all these transactions separately.
OlyZaps transaction may appear more expensive, but that is only because OlyZaps bundles all these transactions into a single, big transaction. If you add up the gas fees of these individual transactions, the math should work out about the same.
As mentioned in the previous entry, OlyZaps may end up costing less because you are not exposed to gas price volatility.