Hi
You need a high temperature glue, it must remain flexible when cured. A number of companies make a suitable adhesive, this is a common problem and you can buy a kit from Oven Door Glue, glass oven door repair kit with High Temperature adhesive. to carry out the repair to your oven door.
Do not use ordinary mastic as it will not withstand the temperature of the oven. Normal high temperature adhesive will not provide for some difference in expansion between the glass and the metal oven door.
ACC Silicones make silcoset 158 which is suitable and used by many of the oven manufacturers. This is the adhesive provided in the kit from the ovendoorglue website. It is black in colour and can withstand up to 300 degrees centigrade.
Dow also make a glue that is suitable however be aware that the Dow glue is red in colour not black, and so may show through on clear or slightly clear glass.
Hope this helps.