The last election was problematic as lots of constituencies did not have candidates in place - for all the major parties. So they rushed through candidates and were not as thorough as they normally are, in their selection. There were gaffs and appalling candidates put forward by all. In this case, one of them, actually got elected.
Momentum has a particular issue, in that it was desperate to get candidates through from its faction of the Labour party to change the direction of Labour. They did use the issue of the snap election to aid this - and didn't necessarily vet candidates in the way they should have.
There is already concern over Momentum and infiltration from the SWP (a rape controversy) and links to the Stop the War Coalition (accusations over anti-Semitism), which have both been subjects of controversy and hypocrisy over an agenda which is not consistent with progressive politics. The fact that Jared O'Mara seems to be not exactly contradicting those fears, does suggest that actually Momentum as a whole, does have a problem, which they are not being honest with themselves about.
This all seems to be known to Momentum members and yet he still got the nod. Why?
Don't forget Corbyn, personally, is strongly associated with Momentum, the SWP and the Stop the War Coalition.
This is really what a lot of the Tory involvement is about imho. The fear of the far left rather than denouncing MPs who behave in an appalling fashion. This will have more traction with an audience which didn't blink at the IRA comments. The Labour Party and Momentum have left themselves open to this, by forgetting the principles they are supposed to uphold. Its the new dead cat attack. They've fucked up big time here. Similar things levelled at Tories don't stick as their voters think in different ways - it doesn't loose them votes.
The reality is that Labour needed to be shit hot on this and they weren't. The danger is it will damage the party. The Tories know it. Especially if O'Mara decides not to resign as an MP. Withdrawing the whip doesn't solve the issue which remains as a reminder as long as O'Mara stays MP.