I have proved the following statement and I would like to know if I have made any mistakes:
$I_1,I_2,\dots$ disjoint sequence of open intervals $\Rightarrow |\bigcup_{k=1}^{\infty} I_k|=\sum_{k=1}^{\infty}\ell(I_k)$
($|\cdot|$ denotes outer measure)
My proof:
By definition of outer measure we know that $\sum_{n=1}^{\infty}\ell(I_k)\geq |\bigcup_{k=1}^{\infty}I_k|$. Now, since $\bigcup_{k=1}^{N}I_k\subset \bigcup_{k=1}^{\infty}I_k$ for $N\geq 1$, from the fact that outer measure preserves order we have that $|\bigcup_{k=1}^{N}I_k|\leq |\bigcup_{k=1}^{\infty}I_k|$ for $N\geq 1$ which implies that the following chain of inequalities holds: $$\sum_{n=1}^{\infty}\ell(I_k)\geq |\bigcup_{k=1}^{\infty}I_k|\geq |\bigcup_{k=1}^{N}I_k|$$ So, if we prove that $|\bigcup_{k=1}^{N} I_k|=\sum_{k=1}^{N} \ell(I_k)$, by taking the limit we would have the desired claim.
We proceed by induction on $N$.
The base case $N=1$ follows from the fact that $|I|=\ell(I)$ if $I=(a,b)$, $a,b\in\mathbb{R}, a<b$.
Suppose now that the claim is valid for $N\geq 1$: we prove it for $N+1$.
First, $|\bigcup_{k=1}^{N+1}I_k|=|\bigcup_{k=1}^{N}I_k \cup I_{N+1}|\leq |\bigcup_{k=1}^{N}I_k|+|I_{N+1}|\overset{\text{ind.hyp.+def.length open interval}}{=}\sum_{k=1}^{N}\ell(I_k)+\ell(I_{N+1})=\sum_{k=1}^{N+1}\ell(I_k)$.
Now, it remains to prove that $\sum_{k=1}^{N+1}\ell(I_k)\leq |\bigcup_{k=1}^{N+1}I_k|$. So, let $U_1,U_2,\dots$ be a sequence of open intervals whose union contains $\bigcup_{k=1}^{N+1} I_k$ and for $n\geq 1$ let $J_n:=U_n\cap (-\infty,a_{N+1}),\ K_n:=U_n\cap (a_{N+1},b_{N+1}),\ L_n:=U_n\cap (b_{N+1},\infty)$. $K_1, K_2,\dots$ is a sequence of open intervals whose union contains $I_{N+1}=(a_{N+1},b_{N+1})$ and $J_1,L_1,J_2,L_2,\dots$ is a sequence of open intervals whose union contains $\bigcup_{k=1}^{N} I_k$. Thus $$\sum_{n=1}^{\infty}\ell(U_n)=\sum_{n=1}^{\infty}(\ell(J_n)+\ell(L_n))+\sum_{n=1}^{\infty}\ell(K_n)\geq |\bigcup_{k=1}^{N}I_k|+|I_{N+1}|\overset{\text{ind.hyp.}}{=}\sum_{k=1}^{N}\ell(I_k)+\ell(I_{N+1})=\sum_{k=1}^{N+1}\ell(I_k)$$ and taking the $\inf$ on both sides we have $|\bigcup_{k=1}^{N+1} I_k|\geq\sum_{k=1}^{N+1}\ell(I_k)$, as desired.
Thank you.
One way to proceed is to note that the outer measure of a compact set $K$ is equal to the Darboux upper integral of $\chi_K$, $U(\chi_K)$, aka the Jordan upper content of $K$, due to the finite subcover property for compact sets. Let $\varepsilon > 0$. Choose $N$ large enough and closed intervals $J_k \subset I_k$, $1 \leq k \leq N$, such that $\sum_{k = 1}^{N}l(J_k) \geq \sum_{k = 1}^{\infty}l(I_k) - \varepsilon$. Then appeal to the result that the Jordan content of a disjoint union of a finite number closed intervals is the sum of their lengths, which is simple to prove using theory of the Darboux integral. This proof has the advantage that it does not assume beforehand that $|I| = l(I)$.