If you're talking about maintenance on a flooded housing I would probably suggest a complete overhaul, unless you know exactly how & where the water got in. If you know this, then replace that o-ring, or solve the problem if it was operator error.
The o-rings are metric JIS (Japanese Industrial Standard) sizes, so do not try to substitute US sizes. Not sure if the control shaft o-rings are the same for the PT-020 as for the PT-015, but here is some info. At least the sources should be good for different sizes of JIS o-rings. All the PT housings are made by the same company, so one would think alot of the same sizes would be used, but you need to check this out. The following is for the
PT-015 housing:
PT-015 O-Ring Sizes
Here's a URL to the thread that contains all the information on the c5050 orings:
http://www.digitaldiver.net/yabbse/index.php?board=2;action=display;threadid=5433
Japanese JIS 2401 "P" series is the Standard for sizes P3 & P5, you'll find the standard here:
http://mdmetric.com/or/jispgb.htm
All o-rings BUNA 70 (aka nitrile)
buttons:
seventeen o-rings, order number OR-2.8X1.9 from Maryland Metrics ( $3.94 per 100)
(aka Japanese Standard Size P3)
http://mdmetric.com/
on/off & mode dial control:
one o-ring, order number OR-4.8X1.9 from Maryland Metrics ( $3.94 per 100)
(aka Japanese Standard Size P5)
one o-ring, Precision Associates custom o-ring 0.053 x 0.074 (unknown cost)
http://www.precisionassoc.com/customf.htm (1-800-394-6590)
Use a good quality generic silicone grease on the Buna (small black) o-rings. The main silicone (red) o-rings probably have to be ordered from Olympus. The do not need to be greased. Just use water or spit to lubricate. Even the "special grease" will damage the silicone material over time. Grease will absorb into the material, making it more pliable & elongate to the point where it will no longer fit the the groove properly.
Hope this helps some. Good luck.