Or In A Join Clause

I'm joining one table on to another table using one of 2 possibile fields (so table 1 key one can either match table 2 key 1 or key 2)... When the first key is null for a record, the script is to attempt to join using the second key instead. It is possible to have both values present, in which case the first one should be used.

I've taken a few runs at this so far:

...
from table1 t1
left join table2 t2
on
(t1.key1 = t2.key1
or
t1.key1 = t2.key2)


If either t2.key1 or t2.key2 are populated, this works. Unfortunately, it's bringing back multiple records if both key1 and key2 are populated. Question # 1... Is there a different relational operator I can be using instead of OR that would logically look like 'if thie first key didn't find anything try the second instead'?


As an alternative, I've put the NVL to use...

NVL(t2.key1, t2.key2) = t1.key1

That seems to work, but it's pretty heavy on the server. Any suggestions on how else to handle this scenario would be greatly appreciated


(and ya, I didn't design this datamodel).