Onegai shimasu vs o kudasai?

Say I have a sentence asking for sushi:

Sushi O Kudasai.

In this case, is the O kudasai interchangeable with onegai shimasu?

f4senkyo69 said:

Ever played Virtua Fighter? That was my achilles heal in Japan. One of the characters victory taunts was 天下無双.

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Yes, in that case, they are interchangeable.

Hmmm... not exactly.
If you mean Sushi wo as in the direct object marker, it needs to be Sushi wo onegaishimasu. Drop the wo, and you can use them interchangeably. Grammatically this works I believe only b/c the 'wo' is implied.

I'm not the best at explaining such grammatical details, but I do know it sounds more natural.