Occupation of Flat without OC, Interest on delayed payment

Booked a flat in Panchkula Sector 20, in May'2010, paid 15%. Builder promised subvention for 2 years, but not materialized. Paid 10% more and got allotment letter. Builder issued permission of mortgage in Oct'2011, alongwith NOC dated 03.06.2011. Applied for home loan. By the time builder issued few demand letters and started imposing interest due to delayed payment. Wrote to builder and builder issued rest demand letters mentioning interest on late payment as "0", till july'2012. Till then 90% amount was paid. Promised delivery period was 3 years from agreement, dated 09.08.2011. On 12.07.2012, builder asked for enhanced EDC, 257150/-asked by DTCP from some developer.. We asked EDC was asked not from you. But there is no reply. Much to our pursuance builder gave possession in Aug'2015 without OC. On 06.02.2016, builder offered OC with late payment 623717. We are residing in the same flat. Pl. suggest what to do? Regards