The other answer is incorrect as OHKO moves are not affected by accuracy modifiers, including items, starting from Gen 3. Zoom Lens did not exist in Gens 1 & 2, so those Gens are out of the picture.
OHKO moves automatically fail if the target has a higher level than the user. Their accuracy starts at 30% and increases by 1% for each level the user is above the target. Accuracy and evasion stats no longer influence the move's accuracy, including modifiers that do not use stages, such as Gravity, Wide Lens, and Compound Eyes.
As both Pokemon are level 50, the accuracy of OHKO moves should be 30%.
However, if in a Gen 7 and above game, a non Ice-type Pokemon uses Sheer Cold, its accuracy is 20%.
Since Generation VII, the accuracy of Sheer Cold starts at 20% instead if the user is not Ice-type.
Hence the accuracy will be 30% for all OHKO moves in Gens 1-6, 30% for all OHKO moves other than Sheer Cold in Gens 7+, 30% for Sheer Cold used by an Ice-type Pokemon in Gens 7+ and 20% for Sheer Cold used by a non Ice-type Pokemon in Gen 7+ in the given situation.
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