Attempting to tune a Sharepoint site using SQL Server Waits and Queues
As this is SharePoint your hands are tied.  You can't create indexes or statistics without loosing support from Microsoft.

CXPACKET just means that one thread of a parallized query is waiting for another thread to finish waiting for something else.  SOS_SCHEDULER_YIELD can mean a few things, typically it means that you need to fix your indexes are you have queries that aren't tuned correctly so the SQL Server is pausing those queries (yielding them) so that it can work on other stuff (as this is SharePoint you can't really do anything about this).  OLEDB means that the SQL Server is waiting on the OLEDB driver so probably there are linked server queries or something along those lines that are calling out to another database server.  There's nothing you can do about that either.

From what it sounds like there's about nothing you can do without throwing more hardware at the problem.  Use Profiler, Server Side Tracing or Extended Events and find the queries which are taking to long to run and check their execution plans and see how bad that are.  Odds are those search queries are doing large scan operations.

How big are the content databases?  Microsoft recommends keeping the content databases under 100 Gigs so that scanning is kept to a minimum.