How memory controller reads from RAM with O(1) time complexity?
The RAM makes use of direct-memory-access, briefly this means that 


  An element of data or instructions (such as a byte or word) can be
  directly stored or retrieved by selecting and using the
  locations on the storage media.


Now this can be happened with the use of a DMA controller, which bypasses CPU to transfer data directly between I/O device and memory. 

If you want to understand precisely how this happens I suggest you read the book 


  Operating System Concepts 10th Edition (Chapter 13)


I hope this helped you to understand why RAM gets instant access, comment below if you have more questions.