A properly designed O-ring seal will be your best option, though for 200C operatiing temperature, you'll need O-rings made from a material than is designed to function at that temp, like Silicone or Fluorocarbon (FKM) rubbers. O-rings are popular because they work well in so many applications and they're cheaper overall than anything that functions as well.
With any O-ring seal, static or dynamic, the key parameter is "O-ring Squeeze" which is defined as the percentage reduction in cross sectional area when compressed from the free state. Since the o-ring acts like a spring, this squeeze is directly related to the sealing force. The greater the squeeze, the greater the sealing force, and the greater resistance to leakage...at least in general.
As with most things, more is not always better, too much squeeze can result in a poor joint since it can cause excess stress in the mating parts, excess friction (in the case of a dynamic seal), and O ring damage since there's a greater risk of the o-ring getting pinched between the two mating surfaces.
In designing O-ring seals in stainless steel with good surface finish, I would try to stick to < 15% squeeze for dynamic seals and 15%-30% for static seals using a standard NBR O-ring from Parker, but it really depends on the type of O-Ring, the materials involved and all of the other specifics of the application. In this case, you'll want to look at the properties of whatever material you choose and as with any quasi-dynamic joint like the one you describe, smooth mating surfaces and lubrication are essential. (I recommend Krytox grease, pricy, but a little goes a loooooong way. You just need enough grease to make the O-ring look wet.)
There's a lot that goes into the design of a good O-ring joint. The best resource I've found is the Parker O-ring Handbook
Source: I designed and tested engineered mechanical seals for 5yr