Oil companies may be rebuilding their stocks of home heating oil in part by withholding or delaying shipments to local distributors, according to a report by House staff investigators.

A survey of Iowa dealers by a House small business subcommittee showed that substantial numbers of them are running seriously behind schedule in their summar fill-up of home heating oil tanks.

The report concludes that the increases in heating oil stock at the company level - the cause for renewed optimism at the Department of Energy - may be artificial. Oil companies may be building their own heating oil inventories at the expense of local dealers, the report says.

The findings cast new doubt on the administration's assurances that the home heating oil supply will be adequate this winter and on President Carter's pledge to have 240 million barrels of heating oil in storage nationwide by October.

The subcommittee report concludes that DOE's optimistic projections for the coming winter are based on statistics showing the amount of heating oil in company storage tanks. Those figures are "unreliable," a ccording to staff director Marc H. Rosenberg, who prepared the report, since they do not take into account shortages at the wholesale, or dealer, level.

"While it may appear that the national stockpile of distillates now is growing rapidly, this may be offset by a growing shortage of fuel at the local level," the report says. "As national energy policymakers seem to be focusing only on national inventory statistics, we may be heading towards a rude awakening....

"A dry August, raising diesel fuel requirements for irrigation in the Great Plains...a wet autumn, compressing the harvest season and creating peak demands for diesel in the Midwest...an early frost in New England or the Middle Atlantic states, creating early demand for home heating oil...these are some of the possible scenarios that could result in spot shortages in diesel or home heating oil in the next few months."

An Energy Department spokesman said that when DOE set a goal of 240 million barrels of home heating oil in storage by October, it decided that company stocks had to be built up even if this resulted in some local shortages.

DOE is now trying to determine the severity of the shortage to dealers, the spokesman said.

The subcommittee staff report cited a Washington Post article reporting that supplies of heating oil in the metropolitan area are so low that local dealers are falling behind in their usual summer fill-ups and some dealers are refusing to take new customers. Distribution from oil companies to area dealers has been cut as much as 65 percent, The Post reported.

The survey of Iowa dealers also showed that 76 percent of those who responded to the subcommittee's inquiries reported significant shortages in their inventory levels, with little prospect for catching up in their summer home tank fill-ups.

A survey being taken by the National Oil Jobbers Council of 6,000 independent dealers nationwide is expected to find the same results - that local distributors are running behind schedule and below inventory. Results of the survey are expected today or tomorrow.

The subcommittee's survey results may be biased, Rosenberg said, since only dealers experiencing shortages would have been likely to respond. Still, the staff director said, the survey does prove that at least 10 percent of the Iowa dealers - and probably many more - are experiencing severe problems in their heating oil supplies.

Local distillate fuel dealers came out of last winter with lower-than-average inventories and then were forced to divert some diesel fuel for agricultural areas experiencing shortages at the time of spring planting.

The diversion caused spot diesel shortages elsewhere that prompted a major strike by independent truckers.

DOE reported 164 million barrels of heating oil in stock as of July 27, compared with 180 million at that time last year. Refineries would have to increase supplies by 8.5 million barrels weekly to meet Carter's 240 million-barrel goal.