How is OH able to stay in USA without the need to apply for greencard?
The most he would stay is a year to 3 years.
If he apply for greencard, can someone tell me what are the steps I basically have to do in order to get a greencard.
In order to live in the US, he will need a visa/green card in the UK before he moves. There is no way for him to move to the US without one.
He can visit you in the US for a maximum of 90 days, but that's it. If he wishes to stay for longer he will need a visa.
The most obvious visa to apply for would be a spousal visa, although you will need to meet the income requirements for it, which is that you need to be earning 125% above the US poverty line (either you, or you can use a family member as a co-sponsor).
However, it will take about 10 months for you to get a spousal visa for him, and for him to be able to move to the US. If you are both in the UK, you can apply using DCF Filing through the US Embassy in London.
First, you need to file a petition to sponsor his visa, which can take 2-4 months to be approved, and then after that he will need to attend a medical exam and visa interview in London.
The whole process will take at least 6 months, generally 8-10 months, and you are advised to file the petition at least 10 months before you wish to move to the US.
If you don't apply for a spousal visa, then your husband would need to find another visa to qualify for to move to the US - either a work visa or a student visa.
You can find out more about applying for a spousal visa for him here:
Immediate Relatives visa:
http://london.usembassy.gov/immigrant-visas/immediate-relatives.htmlFiling a Petition in the UK:
http://london.usembassy.gov/dhs/uscis/i130filing.html