I put one in a neighbours Ash tree a couple of years ago in early spring.Within 4 days it was occupied by an owl.We positioned it as high as we could,and faced it eastwards away from the prevailing wind/rain.Obviously if your putting it in a public area it needs to be high enough to deter Joe Public from trying to climb up to it.You should also place an inch or so of peat in the bottom,as owls don't add any nesting material(apart from pellets they regurgitate).This helps when the birds lay their eggs.Good luck.
Advice required please. Stockport Council have designated Wright's Wood (near Adswood, I think) as a Local Nature Reserve and have asked me about the siting of a tawny owl nest box there as they have one spare (a box not an owl). What are the 'rules' for positioning of such a box. Also, am I being pessimistic in saying that it is already too late for this season?