KCET

Yes, you can still enroll in a college through KCET if you flunked the Kannada language test but only for non-Kannada medium or non-Karnataka candidates.

Here's the simple part — the Kannada language test under KCET is only for Horanadu and Gadinadu Kannadiga candidates (that is students who studied outside Karnataka yet claim Kannada origin). If you don't fit these categories, that test doesn't matter to you at all.

If you took that test to gain the Kannada quota seat and failed the test you wouldn't be able to get a seat under Kannada quota, but if your rank and eligibility meet, then you would be able to get a seat under general merit (non-Kannada).

If you were an Overseas Citizen of India (OCI) and had attended school in Karnataka from Class 1 to 12, the answer is yes you were eligible to apply for KCET 2018, if you met the domicile and education eligibility requirements determined by the Karnataka Examinations Authority (KEA).

The main point to remember is that for KCET, eligibility is primarily based on your academic study in Karnataka and/or your parents' Karnataka citizenship, not quite as much about Indian citizenship. OCI and PIO candidates were considered to be foreign nationals, but if they attended their entire schooling (Class 1-12) in Karnataka, they would typically be allowed to appear for the KCET entry exam under either the Karnataka or non-Karnataka categories (they just may not have been put into consideration for government quota seats).

Yes you would be able to apply for KCET 2018 as an OCI who studied Class 1-12 in Karnataka.
 However may not necessarily have been considered for reservation or government quota seats, only the general merit (non-Karnataka - other) seats.