It was all dubious to me. I generally understood that Oliver spent all of his "cash" money buying up shares in QC and as such, he was cash-poor but still worth plenty because of his shares.
But, I didn't understand how Isabel made Oliver's shares worthless (without making ALL of the shares in the company worthless). I guess she could have simply done that, but there were also background TV reports about QC doing better than ever with Isabel as the CEO following Moira's death. I don't see how diluting the value in QC shares would be a good thing for QC.
I also wasn't sure what happened to Moira's personal fortune. Did she have ALL of her money invested in QC shares? Surely she must have had some cash and some other assets (e.g. property) in her own name which would have been bequeathed to Oliver and Thea. Unless Moira hadn't updated her will since Oliver's return and it all went to Thea. But, that still wouldn't explain why Thea was broke. Of course, it also doesn't explain why Walter wasn't willing to help at least Thea with some money. He'd been her "father" for most of her life, yet he seemed to ditch her pretty quickly and easily because of something her mother had done.
Whatever happened to Oliver's fortune, I'd be happy for Oliver to simply use somewhat nefarious means to fund his operations. Felicity has been shown to be able to easily manipulate the bank accounts of various baddies and I'd have no issue with her transferring funds from known murderers, terrorists, etc into a "Team Arrow" fighting fund. After all, Oliver, Diggle, Felicity and Roy (and Laurel) can't really be expected to work 8am to 6pm "day jobs" to support themselves (i.e. pay for food, clothes, accommodation, crime-fighting gear, super-computers, medical expenses, etc) and then fight crime all night long and still function as human beings.