Be careful with this, as dictionaries are not really O(1), in the worst case, it can be actually O(n). When your code heavily evolves around the 'fact' that it's O(1) try to make a version with a simple array iteration and compare which is faster.
Very approximate and dependent on the case obviously, but I always find it surprising how far you can go with just a simple array.
From 100 to 1000 items the array performance is not massively worse and shouldn't matter much but a dictionary can beat it (by a tiny bit).
If you're over 1000 definitely use a dictionary or tree.