Depends on what you have invested in since the beginning.
If you have a high level of food and steel, and/or have invested in air, naval bombers are good to patrol your coast. They are good at their job, and very good at redeployment, to counter multiple threats. If you have focused goods and oil, and/or have heavily invested in naval units, focus on and produce cruisers and submarines. Don't bother with battleships, they will take too long to produce (however, you are Allies doctrine, so I guess if you want you could. Wouldn't recommend though.) I'm assuming that since you say Ontario is a gold user, they will use either all battleships or all subs, so investing in destroyers isn't bad either. I feel like they probably declared war to either take over your Pacific islands or since they see you as a threat. Be aware that Hawaii and the other islands might be take over before you have a chance to respond.
If Ontario manages to get a foothold, make sure it is not a city, because if they do, they will just gold up a strong defence. Don't worry too much about them taking rural provinces, they won't do much unless they produce resources, just don't let Ontario touch any cores. Station your troops at coastal cities, and keep ranged units (naval or otherwise) to ward off any battleships or cruisers. Note that battleships excel at coastal bombardment, so station some subs nearby incase the battleships are unescorted. Make artillery (I hope you went SP Artillery, because of their Allied boost), place meatshield units in front of them (anti-tanks, tank destroyers, militia, and armoured cars), and bomb any troops that have landed. If still in convoy form, use naval bombers. Get Papua and Borneo to help as well, convince them to send some units to garrison some of your provinces. Tell them to be cautious as well, since they might be invaded as well if you are in the same coalition. Hold a defence until at least a portion of your field army can reach the Americas.
Biggest thing, I advise caution since he is a gold user. He can blow up your industries if he has the brains (and the money), so try and negotiate a ceasefire. Give him some concessions if necessary. Don't wait too long to take him out as well, since time is his biggest advantage.
If you have any more questions, don't hesitate to ask. Hope this info helps 
