How does one combat a GW china as oceania?

How?

You need to especify more your situation, because there are many different solutions based on especific strategies, countries, money, etc.

Are you talking about a player holding all chinese territories? If so, and you have limited income, I suggest a multiple approach with sneak attacks on Japan, Koreas and northern chinese cities while trying to hold a defensive position on Thailand.

Fighting for expansion on Thailand, Vietnam and Burma is a bad tactic, since he will grow larger than you in most situations. Also, expanding to Australia is a must, since it will provide you with the income needed to keep up on fighting long enough to send a counter offensive, but it won't help if you remain stagnant on southern Asia.

Im using Imperialist. Currently, I get about 1.5k per turn and am hovering around 3k total cash. Im not sure about him, but he does own all of china.

I suicided into china southeast with a stealth attack of 16 marines, 8 subs, 5 stealths, and a sentry plane. It was probably in vain, but oh well. it was fun.

Fighting a good GW china is a lost cause unless you use your money and troops correctly. only way i think you can beat China is by taking his citys in China(prefebly caps) and try taking multiple at a time so you have a chance of holding the citys example take his inner countrys since hell be expecting a attack from Coast not inside of china itself but be wary you will need to spend on NOTHING else since marines are expensive and only 2 (3 if you have upgrade or MoS) can fit in one sub so if your Imperialist youll be spending 130 (120 with -10) on Marines and 170 on Subs so youv spent nearly 1k on a couple of marines and subs and if your intial attack is a succes youll probably have a chance against China (as long as no one else plans to attack you).

Sub japan.