现在是F1，已经申请OPT（10月生效）找工作时在公司主页填写简历时会问：是否合法在美国工作？应该填是还是否呢？如果填是，算不算cheating? 菜鸟一问，多谢JM回答！

should be YES! because you can legally work under OPT.

恩，多谢MM，我就是怕如果以后提出办H1的时候对方觉得我没有如实回答

Technically, F1, F2, H1, H4 and J type visa holders are not authorized to work in US. Some big companies like IBM will list the visa type out so you will not be confused. I would say if the company are well known and big, you can answer the question pretty simply (i.e. NO) if they did not specify the applicant must be citizen or gc. Most of them will apply H1b and gc for you in future. In case of small company, you should explain to hiring manager and HR how are you going to deal with the visa problem (opt, h1b...). Some of them just don't want to be bothered with all the paper works.

You should fill "Yes". Most  companies do not know much about the visa details. It does not cost them anything if you hire your own lawyer to apply H1B, which is quite different from the EB green card application. Even you ask them to sign some paperwork in your H1B application, I believe it won't be an issue as long as you are a good employee. On the website of the company I work for, it specifically says: Do not support visa applicaiton but they only mean financially.

should be yes. And luckily you would have no gap since your opt starts from October. Good luck on the job hunting!