How does changing a job work with F1 OPT?

I am currently in an F1-OPT status and this is my first year. I applied for the H1B this year with my Bachelor's degree, but it didn't get picked. Next year, I will be in the Master's bracket and I am expecting it to get selected eventually in 3 years since I have 3 more tries. My question is if I don't get picked next year and if I want to change job, how would that work? If I am in F1-OPT, can I just leave my current company and start at a new workplace immediately without doing anything? Also, if I get picked for H1B in my current company and if I want to transfer to another company, do I have to wait until October (when the H1b gets valid) and then apply for H1B transfer? It would be great if anyone can answer these two questions. Thanks!