Token,
He is using the "O" as an alias. In this case it is not important but if he were to join two or more tables that each had fields with the same name, "ID" for instance then there would be no way for SQL to know which table you are refering too. by setting an alias for each table it allows you to easily distinquish between the tables. For example:
select a.id, a.name, a.asset, m.id, m.model from assetdata as a
left join mdl_tbl as m on a.mdl_id = m.id
Or you could just type the whole table name each time like:
select assetdata.id, assetdata.name, assetdata.asset, mdl_tbl.id, mdl_tbl.model from assetdata
left join mdl_tbl on assetdata.mdl_id = mdl_tbl.id
But the first example is easier to read and it gets worse if your are joining many table together.
Hope this helps to explain it a bit.