Yes, Oracle service Bus is a stand alone service bus.
Before: BEA ALSB -> Now: OSB 10g
Before: Oracle ESB -> Now: Oracle ESB
When you install SOA Suite 10g, you only get the Oracle ESB but not OSB.
When you install OSB 10g, you only get OSB 10g, without any SOA suite.
Before: OSB 10g -> Now: OSB 11g
Before: Oracle ESB -> Now: Mediator
When you install SOA Suite 11g, you now get the Mediator, which is based on the former Oracle ESB but acomplish an SCA (Service Component Architecture, standard) mediation capability but not OSB.
When you install OSB 11g, you still only get OSB 11g, without any SOA suite.
This standalone bus (OSB 10g) can be used for other SOA suites as well (other than Oracle SOA). It means, one OSB 10g installation can be used for Oracle SOA suite and other SOA (from different vendors).
OSB 11g is a seperate product.
So on installation you can decide yourself whether to re use an existing ORACLE_HOME (and so install it in for example an existing soa suite installation) or create a seperate installation for OSB only.
After that you can still integrate with the soa suite or just integrate with your other third party applications.