The Montgomery County Housing Opportunity Commission has opened the way for 500 first-time home buyers with moderate incomes to obtain mortgages with an interest rate of 10 7/8 percent--about three points below the going rate on home mortgages in this area.

The money to finance the mortgages comes from the sale last week of $32 million in tax-exempt mortgage-revenue bonds, a commission spokesman said.

To be eligible for the program, prospective buyers must have incomes in the following range: $27,900 maximum for a household of one, $32,900 maximum for a two-member household, $33,900 for a three-member household, $34,900 for a four-member household and $35,900 for a household of five or more.

The houses available for the 30-year, fixed-rate mortgages are in subdivisions in the county and are priced below $70,000. Prospective buyers must go directly to the subdivision sales offices.

The subdivisions and their developers are:

Greenfield Commons, Ryan Homes, Germantown; Lakeview Condominiums, Pulte Home Corp, Germantown; Castlegate, Pulte Home Corp., Silver Spring.

Peachwood, The Ryland Group Inc., Silver Spring; Oak Springs, Ryan Homes Inc., Silver Spring; Longmead Crossing, Pulte Home Corp., Silver Spring; Tiers of Silver Spring, Curtis F. Peterson Inc., Silver Spring; Goshen Run, Pulte Home Corp., Gaithersburg; Olde Seneca Woods, Crystal Rock Corp., Germantown.

Avonshire, Curtis F. Peterson Inc., Silver Spring; Shady Grove Station, Ryan Homes Inc., Gaithersburg; Nathan's Hill and Essex Place, Kettler Bros., Montgomery Village; Waring Station, Porten Sullivan, Gaithersburg; Tiers of Wheaton, Curtis F. Peterson Inc., Wheaton; South Ridge, Pettit and Griffin, Gaithersburg; Meadowvale and Westleigh, Ryan Homes Inc., Gaithersburg; Middlebrook Commons, Ryan Homes Inc., Germantown.

Also, Greenfield at Brandermill, The Artery Organization Inc., Germantown; Damascus Valley Park, Ryan Homes Inc., Damascus; Wexford, Wexford Development Corp., Germantown; Quail Ridge, Ryan Homes Inc., Germantown; Linden Walk, Holly Construction Co., Silver Spring; Farmingdale, Porten Sullivan, Germantown.

Northlake, Ryan Homes Inc., Germantown; Waverly at Waters Landing, Waverly Homes Inc., Germantown; Greencastle Forest, Ryan Homes Inc., Silver Spring.