Right now front running is very difficult. You can't just do it by offering a higher transaction fee, because Optimism transactions are priced by the sequencer. In theory, you could do it by breaking into one of the routers between the sending system and the Optimism sequencer and dropping packets, but if you can do that you have better things to do than front running transactions.
Once we decentralize the sequencer, whoever runs the sequencer would be able to determine the order of transactions, so some front running might be possible.