Is it correct to say "or otherwise"?

For example, which sentence is correct:

Study hard, otherwise you'll not have a good job.

Study hard, or otherwise you'll not have a good job

Is the particle "or" necessary or is it redundant?

Thanks in advance.

I would say that both sentences are correct, but formally the second sentence ("Study hard, or otherwise you'll not have a good job") is probably preferred. There is a difference in tone and flow between the two sentences though - the former feels like there is a sharper break between the "study hard" and "otherwise you'll not have a good job" parts than the latter, which feels like there is a more uninterrupted flow from the first part to the second due to the presence of the "or".

In the first (and only the first) sentence, you should use a semicolon instead of a comma, because "otherwise" is not a conjunction. This rule is rarely observed except in more formal writing, but I think people should at least be aware of the rule.

- Kef