I am going cherry picking on Monday coming. My parents are looking after Oshie while I'm gone but the weather has been so hot here my tanks temperature has averaged 28 degrees C, and that's with NO heater on at all!
What temperature would be dangerous for my O and how is the best way to cool it? Should I do a small water change and fill with cold water?

A water temperature of 28°C is perfect for Oscars. I know what you mean about water getting very hot. My water often got very hot in the summer time, often reaching 30°C however this won't cause any harm to Oscars. you obviously have to take into account what other fish you have and make sure they are okay with the warmer water conditions. one thing you do have to remember is when the water does get very hot, you can suffer oxygen depletion in the water so if you've got an air stone, keep that running at all times. If you find that the water does get too hot, then just put cold water in until the temperature does drop down a bit

Another way of cooling the water is to fill used lemonade bottles etc with water, screw the lid tight and freeze them, then just pop them in the tank. I would do this if the temperature gets really warm and the fish start gasping at the surface

Thank GOD it's cooler today! Water is at 26 with no heater. Usually I have it at 27 with the heater.
I looked in the fridge yesterday for a bottle with frozen water as mum usually keeps some in there but they were all used up! So I put some ice cubes in there. Oscar went to investigate but by the time he got to them to look, they were already melting!!!

If you put ice cubes in the water dont forget that they will have some chlorine in them, thats why its best to have the ice inside something. Also you will need loads and loads of them to cool the water down