Sanctions have nothing to do with it. It is all about supply and demand. If the world economy is suffering, people have less money to buy oil - the price goes down. If supply is cut (by sanctions or war), the price goes up.
Recently, we (the world) are having an economic slowdown, worldwide. Additionally, at the same time, supplies are increasing, particularly as American fracking units come online. So, more supply + reduced demand == lower prices.
No one is prohibiting oil coming out of Russia. The sanctions are on financial products (loans) and Russia retaliating by sanctioning western food and a few other consumer products. No oil or oil-products involved.
Ukraine and Russia Sanctions has an overview.
The closest western sanctions come to oil is impacting joint exploration projects and financing for projects. That won't cut production in the near term.
Sanctions on Iran are largely meaningless from an oil supply point of view - Iran just sells to China or India instead, so it doesn't materially affect the price of oil, just moves the buyers and sellers around a bit. In the longer term, the impact is that Iran has trouble increasing production, which has happened and tends to increase the price of oil. But, as you point out, Iran is not a big enough producer that it dominates or controls oil prices. No one is, anymore, you have to look at the whole market, everyone producing and consuming. Both are far more diversified than say, 1970 - China and India are accounting for larger and larger shares of the consumption - which makes sanctions of any kind, by anyone very ineffective. The sanctioned parties have plenty of alternative buyers or sellers.