What is the complexity of Array[n] and Object.value
If you have CPython in mind then yes, the both are $O(1)$.
To get array by index the address is calculated as pointer to first element + index * size of element, and for object, which uses struct it is pointer to struct + offset, where offset is calculated at compilation.

The general runtime of operations depends solely on data structure used internally by given implementation of language (please note that language itself might not indicate what structure must be used and the very same language might have different structures using different vendors). It may be offset table, hash table, balanced tree etc.