why is the word 'of' in capital used there?

Where did you see those sentences? Someone may be capitalising the word to highlight it, perhaps as part of some kind of language example or explanation, but more context is needed.

"of my best interest" is incorrect. "in my best interest" would be correct.

Thanks, the first sentence is from BBC, reporting on today's tube strike in London.

Do you mean the 2nd sentence is idiomatically wrong?

These two 'OF' were capitalized by me, as I wanted to know why they were used in these two sentences. Thanks again for your reply on the 2nd one. However I am sill struggling to understand why 'of' is used in the first sentence. Does it mean that skill level of bus driver?

"that" means "the competency and skill level". So, in full:

He said the competency and skill levels required by Tube drivers exceeded the competency and skill level of bus drivers and as such staff needed to be compensated.

"that" is used as a way of avoiding this repetition.

soonhowDo you mean the 2nd sentence is idiomatically wrong?

Thank you for clarifying the 1st sentence. I always remember a phrase: something is of someone's interest.

Are you suggesting this should be : something is in someone's interest?

Thanks again

soonhowAre you suggesting this should be : something is in someone's interest?

Depending on what you mean, it should be either "something is in someone's interest" (it will benefit them) or "something is of interest to someone" (they find it interesting or relevant).

Thanks very much, It makes sense now. I have one last question regarding to the word 'of'

By 1940, the pilot JC held seventeen official national and international speed records, earned at a time when aviation was still so new that many of the planes she flew were of dangerously experimental design,

Q: Why the word 'of' underlined is required there? Does it mean anyt

It is grammatical to omit the repetitious noun in this type of sentence construction. The of-phrase modifies "planes" which occurs earlier in the sentence.

...many of the planes she flew were [planes] of dangerously experimental design.

Thanks. Got it now. Can I modify the sentence to below, to delivery the same meaning?

many of the planes she flew were dangerously experimental designed.

soonhowmany of the planes she flew were dangerously experimental designed.

No, this is the closest you can get:

many of the planes she flew had dangerous experimental designs.