What to do with an existing hydronic house heating system?

my research I've learned how expensive and inefficient heating oil is

Test and measure.

The most common (in the US) type of home heating systems are forced-air. Fans pull cooler air into the system, past a heating element, and then out into the space to be heated. The heated air eventually warms the occupants by convection.

The less common (in the US) approach is radiant heating. Heat up a surface and warm the contents of the space (including people) by infra-red radiation. This is how most of the heat in the universe moves (e.g. the sun's heat across the vacuum of space to the earth).

Hydronic heating via baseboards has several potential efficiency advantages. It is radiant. Radiant heat heats people not air. The transfer mechanism is liquid not air. Water conducts heat more efficiently than air. This is why fiberglass insulation is fluffy full of airspaces and why it loses effectiveness when wet. Finally, radiant baseboard provide heat along the building perimiter, low-near the floor, and without creating drafts (moving air).

Fuel-oil prices tend to fluctuate more than electricity or natural gas because it is less subject to regulation. It is less subject to regulation because natural gas and electricity run infrastructure in the public right of way.

Finally, expensive is relative. That's why test and measure. Fuel oil can be twice as expensive as natural gas. But that might mean $600/month versus $300/month or $150/month versus $75/month. Those are economically divergent against the cost of changing the system.

Ok, the real "finally." If it ain't broke don't fix it. Having a warm house is 98% of what's important. There are lots of other places to spend money that will significantly improve livability. Many of them are going to be easier to accomplish because swapping out heat systems involves lots and lots and lots of tradeoffs, vetting contractors, and living with disruption of significant construction...and of course, swapping out heating systems doesn't make your house better. The fact that the system is still in use is evidence that it is good enough. Good luck.

What I've heard is the opposite. If you are going to burn fossil fuels for heat, those oil burning boilers are more efficient than most hot water heaters and furnaces. The hydronic system is also efficient at getting heat in the right place.

To really improve on an oil boiler, you could get a heat pump or maybe active or passive solar. If you have a system that generates hot water you could probably run it through existing pipes.

Someone told me they switched to heating oil because the local market had only one local utility for natural gas which was charging too much. With heating oil there are multiple suppliers who can drive a truck to your house and price competition.

All heaters are 100% efficient in that any leaked heat leaks into the house you are trying to heat. I would look at insulation if you are trying to be more green.

My hydronic system is heated by a tankless and costs almost nothing. Would that be a possible conversion?

My cousins use a black roof with water pipes just underneath to absorb heat on sunny days, and circulate it down to a giant insulated basement water tank that functions as a heat sink. They then use that to supplement their heating system.

Fill your oil tank now while prices are low. If you can get the company to let you pre-buy at todays prices, I would do that too.

In addition to all the other comments, radiant heat doesn't dry out your skin like forced air does.

This was a big plus for me.