1. means what it says; in very short terms 64 bit OS can address much larger amts of RAM natively, can be useful for eg Oracle DB
note that if you have a 64bit processor, you can run 32 bit OS if you want (although you prob wouldn't).
Note also that as Oracle Linux is a copy of RHEL, 64 bit will be multilib, meaning a 64bit OS+kernel can still run 32bit SW if you don't have the 64 bit available.
3. without src means what it says, you just get the binaries, which is normally all you need.
The src is for if you want to re-compile/customise the SW yourself. Very unlikely if you are asking this qn
4. bonus answer: As above, OEL is based on RHEL, which is currently on 5.6 for v5.x and 6.1 for 6.x series.
for the relevant manuals