If you have used up your OPT visa and still wish to work in the USA to gain more professional experience, you can apply for a visa change after completing your Ph.D. You can change the F1 visa from OPT to H1B visa. For that, you need to ask your employer to sponsor your visa, and then you can continue working in the USA. H1B will make you a professional employer instead of just training while you work as an OPT visa holder. Visit at- https://www.optnation.com/blog/opt-for-phd-students-usa/