welcome to the board!
The term OLL is short for Off List Lower.
Currently certain AR-15 style rifles and their lower receivers are banned by name on a list written by the DOJ and the Legislature, these are called 'listed' weapons.
Because of a court case a few years ago the DOJ can not ban weapons just because they are similar, they have to mentioned by name on the list. You will see this court case referred to as 'Harrots'.
This has resulted in many brands and models of lower receivers for self loading carbines being able to be purchased here in California. Since these lower receivers are not on the list the term Off List Lower was coined, and then the lazy typst in all of us shortened it to OLL much the same as how 'Assault Weapon' was shortened to 'AW'
While this means that we can build these rifles there are still laws that apply and must be followed. The first step to really getting a grip on this is to read the AW/OLL FAQ at this link: http://calguns.net/a_california_arak.htm
It is not entirely up to date but the pertinent facts are still there.
Well, that is quite likely a much longer answer than was anticipated! But it is a complex issue and one that bodes well or our Second Amendment rights.
Once again welcome to Calguns!