Speaking from personal experience I had an accordion fold oil heater crack and explode with a literal geyser of burning hot oil mere feet away from me and my cat. I would never ever put one in a chicken coop - if it ever broke in an enclosed space with the hens the results would be horrific and much harder to treat than a fire-based burn. If you are concerned about heat in your chicken coop getting a flat panel plastic or ceramic heat plate would be much safer. But whether or not you need to heat the coop depends on the breed of chicken, how well your coop is insulated/ventilated, and where you live. Anytime you put electricity in a coop you do risk fire, and any electrical work should be done by a qualified electrician.
In the event that you do use an oil heater, I would recommend placing it somewhere where the chickens could not jump on it or touch it, such as behind a metal grate, and I would keep the area clear of straw. A corner would be the best location, furthest away from their roosts so that the chickens would have somewhere to escape the heat if it got too much. Most oil heaters I know of did not have settings to lock the temperature at, they just cycled on and off and got pretty hot.