The problem with Yemen is many factors . As you know after the British left Yemen separated in 2 countries , north and south. And they went to war with each other , then they re-united after the northern Yemen leader (current Ali Saleh) tricked the president of south Yemen into it , then he use politics to move him out of the way. Many in the south are still loyal to this other guy , and Ali Saleh has made many enemies. Some of the people in east like Hadramout don't even like being called Yemeni, and many tribes have taken arms against the government. Ali Saleh enemies are not only with in Yemen , but during the 1st gulf war (Kuwait invasion) he cheered for Saddam Hussain which resulted in Kuwait & Saudi Arabia taking economic measures against Yemen , and replacing the Yemeni expats they had working there with other nationals. After long years he reconciled with gulf neighbors and actually got large investments in oil sector from UAE , and relation with Kuwait and Saudi became smooth again. But internally the situation got worse for him as the economy was getting worse, he continuously kept making false promises , and refused to leave office despite massive demonstrations , and despite GCC & US mediation with opposition figures he refused to sign many times over and over. And as you know he has been treated in Saudi hospitals after the wounds he got in the assassination attempt in Sanaa. Many people believe he is the problem , and GCC officals say the reason financial help to Yemen was stopped is because they feel the regime there is corrupt and in the past all the money this regime received did not go to the intended projects to help Yemen improve its position to join GCC , but this is not going to happen with Ali Saleh regime in power. Yemen have applied to GCC membership a decade ago , but it seems Jordan is going to be accepted before the Yemen can , because of these reasons and internal political instability , at least for now.
As to relation with Oman , until the 1950's there was a war between the two. And the area between the Oman/Yemen border until you reach the first Yemeni big town (approx 350~400 KMs) has no asphalt road or petrol pumps what so ever, it is a rough terrain and if you need gas you have to knock on people's houses to buy from them if they are selling. Until a decent highway is built there you will not see much mobility by land between the two , although the scenery is amazing for tourists but none go there, as it is an unruly area. There are more Yemenis working in UAE followed by Saudi Arabia, many of them became citizens as the economic prospects in Yemen are dim. Like you said Yemen has oil , and I believe it exports currently over 300,000 barrels a day , but the pipeline carrying that got recently blown up by tribesmen who feel the Ali Saleh regime is stealing their country's wealth, and it better stay in the ground than get revenue to Ali Saleh and his bunch. Not to mention the people of Aden do not like him to begin with. It seems Saudi Arabian king thinks Ali Saleh is his man , and don't wish to see him gone, which resulted in a lot of bad publicity for Saudi Arabia on Arab TVs especially , the Saudi king is being called bad names because of this.(and obviously giving asylum to Tunisian dictator as well). Qatar is on the extreme other end as Ali Saleh seems to hate Qatar & Aljazeera , and he made many accusations against both in public. This resulted in Qatar pulling out of the mediation talks with the Yemeni opposition.