You can register your marriage in ISA with Indian embassy or consulate u der provisions of foreign marriage act 

certificate would be valid in India 

It's not mandatory if you are not a indian citizen and in future don't require any remedy related to the said marriage in India

You can register your marriage in Indian embassy, there marriage officer in all embassies. Such marriage is absolutely valid in India. 

Yea if you register it in the US it'll be valid in India.

If you are Indian citizens then it would be better to register it here too. 

If you are Indian citizens you can register your marriage later in india 

non registration does not affect validity of your marriage 

OPT (Optional Practical Training) is a benefit available to international students in F-1 immigration status who are enrolled in, or completing, a degree program in the United States.

If you are desirous of marrying the person now residing in USA in OPT visa, if his status is resident Indian  then you can get married i USA and get it registered through Indian embassy in USA before the marriage officer designate in the embassy under Hindu marriage act, you can get your marriage solemnised there and get it registered in India later ion based on the proof of marriage in that country and your Indian identity proofs through a marriage registrar within your jurisdiction in India. .

Since you both are resident Indians, it is advisable to get your marriage registered before the marriage officer in Indian embassy at USA itself.

Then it will be better if you get registered in India

Registration of marriage is a mode of proof of marriage. On adducing the registration certificate of marriage, it may be presumed by the law that marriage is deemed to has taken place.

The Foreign Marriage Act, 1969

This Act to make provision relating to marriages of citizens of India outside India. For the purposes of this Act, the Central Government may, by notification in the Official Gazette, appoint such of its diplomatic or consular officers (an ambassador, envoy, minister, high commissioner, commissioner, charge d’ affaires or other diplomatic representative or a counselor or secretary of an embassy, legation or high commission.) as it may think fit to be Marriage Officers for any foreign country. The requirement for this will be your age proof, (Passport, Birth Certificate) your address proof (Passport, Ration Card, Voters Identity card) and Martial status. The best for you both to give one notice for marriage under the Act of 1954 to the Marriage Officer of the District where you permanently reside in India, after one month notice period marry her in presence of 3 witnesses by the Marriage Officer, get Valid Marriage Certificate from the Marriage Officer.

You can register your marriage in the USA with the Indian Consulate under the provisions of foreign marriage act . 

Once a competent marriage registry issues you a certificate of marriage in the USA, nothing further needs to be done in India. Registration is but optional under the Foreign Marriages Act in India.

- Since , you both are residing in US , then you can register your marriage there under the Foreign Marriage Act after approaching the consulate of India. 

- Further, the said certificate is valid in India  as a proof of legal marriage . 

- Further if there is problem from the consulate , then you can solemnized your marriage under the Hindu custom there , and later on you can register the said marriage in India . 

Dear Client,

                  Every NRI who marries a citizen of India or another NRI must get his marriage registered within 30 days. If the marriage occurs outside India, it must be registered with a Marriage Officer, who will be appointed from among the diplomatic officers in a foreign country.Marriage that occurred outside India is valid but should get registered in india  to make or enforce it as legally valid in India.

Thanks & Regards