It is quite possible to be flying IFR under a radar service OCAS infact I have done so in the high teens down the east side of the UK in business jets and turboprops from Newcastle to the London area.
The same goes on areas to the west. As such pop up traffic is a possibility but you are not going to get much popping up at FL180
certainly not the 150s and usual spam cans.
hence flying high OCAS has to reduce collision risk but at some point you have to come down and then you are mixing it with every type of flying machine available
Anything which will make you feel safe such as TCAS or even a shute system on a cirrus will make you feel complacent but OCAS BEWARE! with TCAS
Had the rude awakening of counting from the ground 11 hangliders floating around the cloudbases in wales at about 2500 feet while driving my car.
Three disappeared for short spells into the clouds which is really a scary thought
Pace