OLAP or OLATP - How to determine
From a DBA perspective, the key difference between OLAP and OLTP is the tuning method you apply to the queries. The read/write ratio doesn't really tell you anything useful.

I have a little "magic quadrant" that I use to illustrate the difference (in your case, consider BI/DW and ETL the same as OLAP):



Basically, if you must touch a lot of data to produce query results, you are OLAP. Converse if you can get away with touching very little data to produce return values, you are OLTP. 

Obviously, if you have poor indexing strategies, then OLTP can end up looking like OLAP - simply because you end up doing table scans all the time. A good way to make that distinction is to look at how much data MUST be touched by the top 10 most important queries in the system - no matter how good your indexing strategy is.

Once you have settled on what the majority of your workload is, you can start applying workload specific tuning techniques. Here are some of the workload specific tricks you might consider (always tailor to your specific workload)

OLAP:


Prefer hash joins
Optimise table scan speed (typically by laying out data for sequential I/O)
Aggregate view are often better than indexes
Column store indexes are better than B-trees
Sequential I/O
Aggressively de-normalise
Use high MAXDOP
Optimise network for bandwidth


OLTP:


Prefer loop joins
Optimise for index seek (there should always be an indexed path to the data)
Rarely aggregate
B-tree indexes
Random I/O
Keep database in 3NF
Use MAXDOP 1
Optimise network for latency