The test is online. will they know if i cheat?

Yes. The test employs proctoring software that watches candidates through their webcam as they complete parts B and C. 

how long are the oacp and tnt certificates valid for?

The certificate is valid for 1 year. Candidates may choose to renew the certificate as early as 10 months.

what is a "good" test score?

Test scores are not disclosed to the candidate. This makes it VERY difficult to know how to improve. In fact, it makes it difficult to even know if you NEED to improve. Practice tests are a good way to evaluate your test scores and improve your performance.