Being possible is different from being so. To show that "okay" really comes from "och, aye," one would need more evidence than a superficial resemblance. For example, none of the following conceivable sorts of evidence have been shown to exist after nearly a century of scholarly exploration.
1. A pre-1839 spelling such as "okay" in the north of England (or elsewhere in the British Isles).
2. A pre-1839 record of a Scottish person in North America saying something (mis)spelled as "okay," "OK," etc.
3. A pre-1839 example from anywhere of anyone using "Och, aye!" as an adjective meaning "correct, good, excellent," which is what early examples of "OK" do mean. Instead, "Och, aye!" is solely an interjection.
4. A comment, recorded shortly after the appearance of "OK" in print in Britain (1870s) along the lines of "We've been saying that in Scotland for years". This would at least give plausibility to the argument.
One is also curious to know why generations of American scholars might unanimously wish to "cover up" a Scots origin of "OK". Many Scots emigrants settled in America, and citizens of Canada and the United States of Scots ancestry (including numerous scholars) are quite proud of it.
In the absence of documented pre-1839 evidence, any revisionist theory of the origin of "OK" exemplifies what philosophers refer to as "the appeal to ignorance".
Believe as you will.