Yes you can lay your oxygen concentrator on it's side without any problems.
Just make sure that if you do lay the oxygen concentrator on it's side to make sure the vents are not blocked and as long as the vents on the oxygen concentrator are not blocked it's okay to lay it on it's side.
You can lay the oxygen concentrator on it's side whether it's on or off and it will do no harm to it.
Also if you have an oxygen tank never let anyone who is untrained use it because people must be trained to properly use the oxygen tanks for safety and health issues.
Never let children near the oxygen tanks either as they can be harmed and teach them to never touch the oxygen cylinders or play with your oxygen concentrator.