OAEP security with variable length hash function
Beware that if $n &lt; m$, then $\operatorname{SHAKE256-}\!n(x)$ is a prefix of $\operatorname{SHAKE256-}\!m(x)$, so the two functions are not really independent random oracles as the usual OAEP theorems posit.

If you set $G(x) = \operatorname{SHAKE256}(0 \mathbin\| x)$ and $H(s) = \operatorname{SHAKE256}(1 \mathbin\| s)$, that should be adequate to (conjecturally) satisfy the hypotheses of the theorems without requiring additional analysis to study the possibility of collisions between the inputs to $G$ and $H$.

Alternatively, if the inputs to $G$ and $H$ are guaranteed to have distinct lengths in your application, then $G(x) = \operatorname{SHAKE256}(x)$ and $H(s) = \operatorname{SHAKE256}(s)$ should work too.  But it won't hurt, and might be safer to avoid mistakes, if you always use a unique prefix, whether it be a 0 bit vs. a 1 bit, or the string G oracle vs. H oracle, etc.