May not be an easy answer as there are a wealth of stainless steels that will make quality oyster openers. What are you familiar with? Does your heat treater regularly process some of the popular trendy knife steels?
They won't need regular sharpening so that may ease the steel requirement for edge retention uses.
S35, S30, then the old school ATS-34, 154CM and 440-C are which are a bit lower cost to buy.
The PDF for some of these steels are here, click the PDF at the top of the page to download:
https://www.admiralsteel.com/products/blades2.html