Hello Sohil,
For the future it really would help if you specify what revision you are running.
1) Once you reach the 2bn limit then you do have to partition.
2) Once you have partitioned a table each partition can hold up to 2bn records.
3) Your OOM could be due to merge operations but without proper information that's just a stab in the dark.
If you are really interested in a deeper look at the subject there is a great article put together by Henry Cook. 2 Billion Rows is Not a Limit
Even thought the following doc refers to SPS3 it will still give you an idea on partitioning: SAP HANA Database – Partitioning and Distribution of Large Tables
Checking your compression which will impact memory consumption can be done with the SQL in the following note: 2105761 - High memory consumption by RANGE-partitioned column store tables due to missing optimize compression
The above is just some starting point info for you to help nudge you towards a better understanding of what you have.
KR,
Amerjit