If you have a built Omni and you select it to go somewhere it puts down a green icon to indicate where it is headed to. If you then click on the green icon the omni will now follow your mouse and the only way to release it is to place the omni like you normally would. Esc doesn't work to release the omni.
Pretty sure that's intended so you can sneak Omnis into the backfield by manually navigating the obstacle course of particles.