The woman who was arrested denies all charges.
Can 'who was' be omitted from this sentence?
I found the following sentences on the Internet:
The man arrested by the police on suspicion of theft had stolen a pen from a professor's house.
The man arrested by the police, whose identity is not yet known at this stage, is 50 years old.
Mr. Alba, the man arrested by the police yesterday, works at Lincoln Hospital.
In all of the above sentences, "who was" has been omitted. Therefore, I'd say yes, you can omit "who was".
[1] The woman [who was arrested] denies all charges.
[2] The woman [arrested] denies all charges.
Both examples are grammatically fine and semantically similar.
In both cases, the bracketed clause is a modifier in noun phrase structure. The major difference is that in [1] the modifier is a relative clause, while in [2] it's a past-participial clause.
The past-participial clause in [2] is a 'bare passive', as evident from the admissibility of a by phrase in internalised complement function (cf. The woman arrested by police ...).
Note that some people call the past-participial clause in [2] a reduced relative clause, but that analysis is best avoided since there is no possibility of it containing a relative phrase (cf. the ungrammatical *The woman who arrested denies all charges).