As stated in many other of these comparison threads I prefer the Stihl chain, to me prolly has a higher rockwell hardness, thicker chrome and as a result holds an edge longer, as stated by the other poster, it results in less filings and we get longer chain life. I have to spend 1700-2000.00 a year on chain and want all the life I can get. Oregon is a good chain, I think it runs a little smoother, it seems softer, I feel like I can get the oregon a little sharper, but don't feel like it holds an edge like the Stihl. You can run the Stihl chain on your 555 just fine, when I owned my 346 I ran Stihl chain on it all the time.
If your a casual woodcutter, don't discount the Woodland Pro sold by Baileys, I prolly like it better than the Oregon for the money, Baileys has a excellent cross reference chart. For the money it is a great value, made by Carlton in the US.
Main thing is to get rid of any of the safety "anti-kick back" chain. Try you some Stihl RSC full chisel, or RMC semi chisel if your cutting in dirty conditions, the Oregon lpx is also a good chain, the woodland pro full chisel has given excellent results as well. Enjoy the 555, be safe.