Is it possible to launch a DOS by repeatedly requesting a webpage (refreshing)? Like holding down F5 on your browser?
If the page you are refreshing is sufficiently resource intensive. In most cases it will not result in a denial of service as the latency in the network connection far exceeds the page load/cpu time. If you found a page that did significant background work, such as creating a database backup you could in theory repeatedly refresh the page and consume all available CPU/memory/etc.
In principle, yes it is possible to DoS a site by repeatedly refreshing a website in a browser. An example of accidental DoS by browser loads is the Slashdot Effect. In practice, DoS with a browser is not cost effective for an attacker because the cost of doing a page load and rendering the page in a browser usually exceeds the cost of the server for serving the page. Most DoS attacker would use specially designed tool that can make a request without rendering the page, this lowers the attackers' cost.
Generally DoS is the most effective if the target server has some resources that incurs asymmetric resource usage, i.e. the request is a small payload but the response requires thousands of database requests and/or heavy CPU usage. This can easily be abused by an attacker that makes a request then immediately disconnects. If the server isn't well designed enough it may continue producing the page while the attacker is now free to do another request.