See also Terence Tao's comments at https://terrytao.wordpress.com/2022/05/10/partially-specifie... which say things even more strongly (I had collected a link to it at https://shreevatsa.wordpress.com/2014/03/13/big-o-notation-a...):

> The symbol ∈ only is a viable solution in a portion of the use cases. For instance, an assertion such as O(n)⋅O(n) = O(n²) would not be correctly describable as O(n)⋅O(n) ∈ O(n²). Perhaps O(n)⋅O(n) ⊂ O(n²) would be defensible, but now one has to devote a non-trivial amount of thought into deciding which of the connectives =, ∈, ∋, ⊂, ⊃ to use in a given context. For instance the assertion “Since sin(y) = sin(x) + O(|y−x|), we have sin(x+O(1/n)) = sin(x) + O(1/n)” would now become “Since sin(y) ∈ sin(x) + O(|y−x|), we have sin(x+O(1/n)) ⊂ \sin(x) + O(1/n)”. Using the equality sign for all of these use cases instead is more intuitive and corresponds more closely to how the verb “is” (“to be”) is actually used in mathematical English.

and

> … Nevertheless most of us still often think in mereological terms rather than set-theoretic or first-order terms […] without requiring translation to set theory or first order logic; indeed, such a translation would only serve to slow that mathematician down as he or she would usually have translate it back into mereological form in order to wield it effectively. Because of this, I think it is worth adjusting our notational conventions to more closely align with our actual thought processes… I don’t see much advantage in interpreting each instance of the O() notation in the exponential type bound f(n) = O(\exp(O(nᴼ⁽¹⁾))) or the calculation (1 + O(1/n))ᴼ⁽ⁿ⁾ = \exp(O(1/n)⋅O(n)) = \exp(O(1)) = O(1) (for n sufficiently large), in terms of ideals.