In it’s original manifestation it had two broad meanings:
1. Construction of airspace and routes such that the requirement for positive separation actions is minimised.
2. controller technique, whereby, in simple terms, traffic was not placed such that, if the controller was distracted or overloaded, a breach of separation would occur. For example, if you were radar vectoring two aircraft on reciprocal headings then you would apply level stops or slightly different headings, in case you dropped dead.
That was the simple concept, don’t know what happens these days.
LOSA was for cases where an occurrence was less than desirable, but no standards were breached, originally a form of statistical gathering to highlight areas of concern.