If you are going to watch the eclipse then you must have proper eye protection or you will seriously damage your eyes. Eyewear that will let you safely stare at the sun can be ordered here: http://tinyurl.com/solarviewers
On the Oregon coast, temperatures can be chilly within a mile of the coast and get warmer very quickly as you go east. Layers are key. Long sleeves and a wide brimmed hat will make the whole thing more pleasant since even on the coast, the sun can be intense. Mornings in the summer have a habit of being clear and then the fog will roll in during mid-day as central Oregon heats up.
Central Oregon in August can be mild – in the 70’s or as high as 100. In any event, the sun will be intense. A light colored, light weight, long sleeve cotton shirt, a wide brimmed hat, and sun glasses will make the day much more pleasant. Depending on the circumstances, water may be a problem. Bring as much of your own as you can.
Eastern Oregon is mostly high desert. In August, the days can reliably expected to be very hot (100+), very dry, and the sun will be unyieldingly intense. Dressing appropriately will mean the difference between a good experience and a fierce sunburn and possible heat issues. Light colored, light weight, long sleeve cotton shirt, light weight trousers, and a wide brimmed hat will keep you cooler and protected from sunburn. Good sunshades will protect your eyes. Drink plenty of water. You can dehydrate rapidly in the very low humidity without realizing it since the sweat will dry almost immediately. Dehydration will also affect judgement skills when you fly out.
It should have been noticed by now that a reoccurring theme for Oregon is that of sun exposure. The sun in summer loves us on more in the Pacific Northwest. Shade yourself. Cover your skin and it will be very much more pleasant.