The OR statement is implemented a bit differently than people might expect. There is a new query parameter (opmodeor) that you need to enable to treat the oslc.where as an OR statement. And you would still use "AND" in the query parameters.
For example, the following URL:
?lean=1&oslc.select=assetnum&oslc.where=assetnum="11430" and assettype="IT"&opmodeor=1
Would return anywhere the assetnum is 11430 or the assettype is IT. If you need to mix AND statements with OR statements, you should use a saved query or an automation script with search query parameters. I documented an example automation script here: IBM Maximo Autoscripting Guide – Object structure query
NOTE: I realize when I created the page I put "Until Manage 8.3 or Maximo 7.6.1.3, utilizing the oslc.where would not allow you to combine AND and OR statements in the where clause." but I misunderstood what was being added. I no longer work at IBM and can't update this page to correct my mistake.