The Millers' summer recruitment has been in overdrive so far and Evans has already added six players to his squad as he plans a promotion push next season.
O'Kane had a fine campaign for the Gulls last term and earned international recognition for Northern Ireland Under-21s, before deciding to switch allegiance to the Republic of Ireland.
"He is a talented young man and there is more than Rotherham United looking at him," Evans said. "But we are going in a different direction.
"He will probably move on this summer, to higher than League Two, which is what his ability deserves. He is not a player we have bid for."