To remove the Google search bar at the top of your home screen tap the options button while at the desktop and select the three dots above "widgets" then turn Search Bar off (highlighted here in yellow).
The OnePlus uses Cyanogen so this should be the same for any device that also has the same OS version.
I've been looking at server security, testing things on my own web server, and recently found a company called Beyond Security who do free site testing. It looks like they go pretty in-depth with their automated checks and they give you this seal (which doesn't always seem to load so it might look like a blank space):
Their site is a bit flakey and the report you see when you click the seal as a website visitor doesn't really reflect anything but as the site owner you get a detailed list of potential vulnerabilities.
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