What is the difficulty level of CAT when compared to other exams? How much should one study (of OC) to get into the IIMs?

CAT is considered one of the toughest exams which is similiar to XAT and GMAT.
CAT considers normalization method for results.
Apparently, you can join coaching Institute like TIME, Careerlauncher, IMS, CPLC, CATking feom secind year for CAT preparation which can be attemptes only from last year onwards with Minimum 50% in graduation as eligibility criteria.
A minimum of 90 percentile in CAT is must for agribusiness management course in IIM Ahmedabad and Lucknow for PGP.
Once you start with Preparation, you would need to appear for mock tests to gain enough confidence.
You can refer to books by Arun Sharma, Nishit Sinha, TIME Coaching Material for OC sectio.
You can give sectional mock exams fromTestfunda, oliveboard, hitbullseye to identify your weak areas and time consumed.
All the Best.

If you compare it to Exam like CMAT, MAT, IBSAT etc it is significantly tougher. If you compare it with IIFT and SNAP these exams has a different section for general knowledge which is perceived to be difficult by various candidates. XAT is significantly tougher than CAT in terms of level of difficulty.

Although these are different in terms of pattern and hence all of these require an intense practice and different strategy to crack these exams.

For preparation you can refer to following reference books as per your choice: 1) How to prepare for Quantitative Aptitude for the CAT How to prepare for Data Interpretation for the CAT How to prepare for Logical Reasoning for the CAT How to prepare for Verbal Ability and Reading Comprehension for the CAT 2) Pearson Guide to Quantitative Aptitude for CAT by Nishit Sinha Pearson Guide to Quantitative Aptitude and Data Intrepretation for CAT by Nishit Sinha Pearson Guide to Verbal Ability and Logical Reasoning for CAT by Nishit Sinha There are others books like Word Power Made Easy by Norman Lewis, High School English Grammar and Composition by Wren and Martin etc to help you for your preparation. You can also join various coaching institutes like TIME, CL, IMS etc or the online coaching institutes also for concepts and mock tests. All the best!

Depends upon ypur preparation, practice well, and you'll find it moderately difficult only.