In some very limited contexts, yes. As the example you linked to ("The one, the only...") demonstrates, the word "only" can be used to mean "best" in a somewhat indirect way by saying that some person, entity, etc. stands alone in their field, implying that they are the first ever to achieve such a high level of expertise or skill, and thus they are, by extension, the best in their field.
However, in the above context (Harry Potter), no. The word "only" means that the folks referred to by "some o' the best" were literally the only ones in their family lines to have magic abilities; no other person before or beside them had such abilities.
This doesn't imply that they were the best magicians. In fact, quite to the contrary, it seems that some other character (the "he" in "what does he know") has implied that Harry's lack of magical heritage is in fact a very bad thing, not a good thing. (If I'm interpreting this correctly; I know little to nothing of the story.)
However, the speaker does state outright that the characters he refers to are "some o' the best [they] ever saw". So while "the only" doesn't mean "the best", "the best" does mean "the best".
Tyler James Young has already answered some of your questions in the comments. Here I will continue Tyler's work and try to answer the remaining questions as well:
What does the word "trash" mean?
As explained by Tyler, in this context "to trash the quarterly OKRs" means "to ditch/discard/throw away the quarterly OKRs".
What does the phrase "given a brilliant mid-quarter discovery" mean?
The term "quarter" in this paragraph means "a period of three months", that is a quarter of a year.
A "mid-quarter discovery" is a something that has been discovered during a quarter.
"Given" in "given a brilliant mid-quarter discovery" is a preposition with the meaning of "taking (something) into account". In this case (something) is "a brilliant mid-quarter discovery".
Thus, "given a brilliant mid-quarter discovery" could be rephrased as "taking into account something brilliant that happened during a quarter".
What is the exact meaning of the first sentence?
Let's now consider the full sentence:
If you’re finding your team sets quarterly OKRs only to trash them each quarter given a brilliant mid-quarter discovery, you’re either A, B or C
It tells us that if your team sets an OKR every quarter and at some point during that quarter your team discovers something brilliant and decides to discard the OKR, then your team is either A, B or C.
Best Answer
You're absolutely correct. That's exactly what it means: in terms of size, the Taklimakan Desert is second to Africa's Sahara Desert and no other desert on this planet. The adverb only is used to convey the idea that the Taklimakan Desert is second in size exclusively to the Sahara Desert - which means that the Sahara is the largest desert on the planet, while the Taklimakan Desert is the second largest. Only in this sentence is really used just for emphasis. Semantically, it does not carry a lot of weight: