You have prop shaft seals to consider but if like Mercury, the prop shaft is sloped as is the thrust washer inner diameter to ensure that the washer is exactly centered for minimum prop vibration. Nothing is locking it on but brute force (the 55# to which the prop nut was originally tweaked..on larger engines, probably half that on your 25) and probably some corrosion. Heat would help but I wouldn't with the seals. A flywheel puller would work. Harbor Freight has them at a great price.
When you replace/reinstall it, get some marine grease (if you can't get OEM, white lithium is available at WW or auto parts stores) and grease the shaft good, both where the washer mounts and the splines of the shaft......and ensure you match the slope of the washer to the slope of the prop shaft.
Lower unit oil used to be regular gear oil back in the early days but today's engines have a dedicated gear oil. WW has it in quarts with an installation adapter for Mercury engines. No reason that wouldn't work in your OMC if your WW doesn't stock it or you don't have a marine store handy that caters to OMC engines.
