Omen does not charge its own trading fees. The only fees that apply to a trade are those imposed by Polymarket on the underlying market, and they vary by market — many markets have no fee at all.
Where a fee applies, it is not included in the price estimate shown in the order entry box. Fees are accounted for in the order review window, which is why the estimate there can differ from the one you saw a moment earlier. See Order Types and Slippage.
You can avoid fees by letting a position resolve. Positions resolve automatically at the end of an event, and no fee is charged when they do. Selling out of a market instead — particularly after it has closed but before it has resolved — incurs Polymarket's fee where one applies. If you already hold the outcome you expect to win, waiting for resolution costs you nothing.
The only payment you make to Omen is the price of an evaluation. See How Much Does an Evaluation Cost?.